Cast tick label to string prior to `replace` call

In the edge case that an actual integer, or other non-string, is passed as a label, it must be cast to String before `replace` can safely be called.

The recent changes to add retina support included a call to getContext
made before excanvas/flashcanvas had a chance to register their hooks;
fixed by moving that code up a few lines.

- Minor code cleanup and comments describing how we use pixel ratios.

Basic retina display support

Auto-generation of legend colors no longer results in only white and black above a certain number of series.

As the size of the variation increases, the resulting colors approach
white and black. To avoid this we now reset the variation when it gets
too large. This results in repeated colors, but that's much better than
a list full of whites and blacks.

Linked the errorbars example to the examples index page, and cleaned up
both the example and documentation at the top of the plugin.

Added a plugin to generate error bars.

Allow custom highlight color

Correct GitHub Syntax Highlighting in API markdown file

Check whether an axis has a generator, rather than a mode. In most cases
this is functionally the same, but it also handles the case where an
axis mode plug-in specifies only the formatter, and expects to use the
default base-10 generator.

Added an option to control legend sort order.

Added a legend 'sorted' option that allows sorting of legend entries
independent of series order.  It accepts either null/false, true,
'ascending', 'descending' or a comparator function.

Throw error when flot.time plugin is missing.

- The faster toFixed alternative now returns a string, as tickFormatter
is expected to do

We changed the === to == and removed the typeof operator.  We also changed the tabs to spaces.