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".

This resolves issue #964.

Fix example (syntax error and indenting).

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.

Added the plot function as a chainable property.

Modern browser consoles obviate the need for an object logging function.

The previous processing step already guarantees that the values are
numeric.

This resolves #534 by handling the case where data is a string, whether
it is provided alone or as a pair.  We can also eliminate the old and
unnecessary check for undefined values.

Resolves #734 and #816.

Fix inability to set the point lineWidth to 0.

This resolves #895.

This temporary fix addresses the problem described in #810, and possibly
#945, where calls to setData for a pie chart do nothing.

The canvasWidth, canvasHeight, legendWidth and legendHeight variables
existed only to share data between the setupPie and draw functions.  Now
that setupPie has been folded into draw, they're no longer necessary.

This code is only used to prepare the plot for drawing; inlining it is
cleaner and will allow us to get rid of some plugin-global variables
that existed only to give setupPie a way to share data with the draw
function.

The redraw flag starts out as true, then is immediately set to false on
draw.  If labels are enabled, drawPie calls drawLabels, which calls
drawLabel for each label.  Any label that can't fit sets redraw back to
true, so the whole process can repeat.

This isn't the most obvious mechanism, and forces one to remember to do
things like setting redraw back to true after drawing, so the plot can
redraw itself on resize or when setting new data.

Instead we now have drawPie return true when it drew successfully, and
false otherwise, which the same happening in drawLabels and drawLabel.
Instead of checking the flag, we now just check the return value.

This has the added benefit of slightly improving performance in the case
where several redraws are necessary, since it now short-circuits out of
the draw loop as soon as one label fails to fit.

The total is only used within the combine function; it doesn't need to
exist at the top level of the plugin.