This partially addresses #991, but a more complete solution, or better
documentation, is still necessary.

There's no need to continue in the function when all highlights are
being cleared.

The unhighlight method retrieved the point differently from highlight,
and incorrectly.  It now matches the behavior of highlight.

Add an extra pixel to ensure that the lines cleanly join both top and
bottom ticks.  Also apply the sub-pixel offset only in the necessary
direction, so the lines don't start a pixel off from where they should.

The axis color now controls the color of the axis line, instead of its
ticks and labels, while the tickColor controls the tick color. This
makes a little more sense and provides the minor feature of being able
to change the axis line color separately from that of its ticks.  Pull
request #917 ought to be easier to merge now, too.

Plugins are re-initialized with each re-plot (which may not be the right
thing to do, but that's how it works for now).  The previous approach of
saving references to the original Canvas functions therefore broke,
since the second time around we'd get a reference to our new function.
Instead we hold those references as globals within the plugin, and only
set them once.

This whole idea of replacing prototype functions is, now that I step
back and look at it, really awful.  This needs to be changed ASAP to
something less ridiculous.

This allows us to reuse the canvas elements without having to retain the
rest of the Canvas object, which should really be reset when the plot is
reconstructed.  It's also a little simpler, and the Canvas constructor
just feels like the right place for this code.

Update examples link to flotcharts.org.

Add information about assigned issues.

Update copyright year; resolves #927.

Moved canvas text support into a plugin.

These styles are deprecated, but we'll continue to use them until the
release of version 1.0.0, for backwards-compatibility.

The tickLabel class is deprecated in favor of flot-tick-label, but we'll
continue to use it until the release of version 1.0.0, for
backwards-compatibility.

Added selection plugin options.

Tweaked the description to indicate that we no longer combine single
slices into 'other', and reduced the threshold from 10% to 5% to
encourage more slices to combine in the demo.

Resolves #638.

Resolves #489.

Previously, the minimum size a selection could be was set at five
pixels. This commit adds the ability to customize this value.

Previously, if the selected area was very small, the selection
rectangle would not be displayed. This commit adds an "alwaysShow"
option so that, when true, the selection rectangle will always be
displayed. When the selected area is very small, the selection
rectangle will become a line.

This commit adds an option for the shape of the corners of the
selection rectangle. By default the shape is set to "round" (the
previous setting for lineJoin). The other options are "bevel" and
"miter".

Previously the cache was divided only by layer, with entries keyed on a
string built from the text and style.  Now the style has its own tier in
the cache, i.e. layers > styles > text > info.

This introduces some complexity, since the nested for loops are ugly,
but at the same time we avoid having to create the cache-key strings.
More importantly it solves the problem of uniqueness that exists when we
try to join strings that may contain arbitrary text.  It also allows a
further optimization in the canvas plugin, which can now set text style
and color just once per distinct style, instead of with every string.

This lets users 'namespace' text more naturally, i.e. placing x-axis
labels in a different container from y-axis labels, providing more
flexibility when it comes to styling and interactivity.

Internally the text cache now has a second tier: layers > text > info.

The getTextInfo method previously added new text to the top-level
container when measuring it.  Now it adds the text to the text layer,
just as it will be when rendered, so that parent-child CSS rules can
resolve correctly.

This also avoids having to safe a reference to the top-level container,
since it wasn't used anywhere else.

This sets the stage for allowing the use of multiple layers.