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.

Also switch from dashes to pipes, and remove the angle for now, since we
don't currently support rotated text.

Every cache element now contains the actual text element instead of just
its HTML, plus a flag indicating whether it is visible.  The addText and
removeText methods control the state of this flag, and the render method
uses it to manage elements within the text container.  So where we
previously used drawText to actually render text, now we add each string
once, then let the render method take care of drawing them as necessary.

This dramatically improves performance by eliminating the need to clear
and re-populate HTML text on every drawing cycle.  Since the elements
are now static between add/remove calls, this also allows users to add
interactivity, as they could in 0.7.  Finally, it eliminates the need
for a separate 'hot' cache.

I also removed the unnecessary 'dimensions' object; it's easier and
faster to store the width and height at the top level of the info
object.

This resolves issue #964.

Fix example (syntax error and indenting).

This significantly improves performance, since we already create the
elements when measuring them, and that effort is now no longer wasted.
We must take care to detach, rather than remove, when clearing the text
layer, so we can add the elements back later if necessary.

Add basic Travis CI setup with test script that runs JSHint.

This resolves #18, but without catching zero, which is a valid key.

Added a legend 'reverse' sorting option.

This orders the legend entries in reverse order of their series.