Makes hover detail arrow rendering more reliable

Uses the 'CSS triangle' trick to render the left and right arrows on the
hover detail item, rather than using a glyph (whose rendering might
change depending on the font used).

The technique is described here:

http://appendto.com/blog/2013/03/pure-css-triangles-explained/
Add test case for Legend
Make Rickshaw.Graph.Legend a real class

This makes it possible to extend the legend to add custom styling
or interactivity. This commit should not change behavior or options.
Allow Legend subclasses to override the className
Extract legend rendering into render function that can be called multiple times
Return rendered line from addLine
Merge pull request #334 from kungfubanana/hover-detail-arrow-rendering

Makes hover detail arrow rendering more reliable
Merge pull request #339 from dwt/make-legend-a-class

Make legend a class
tweak area renderer to play nicely with multi renderer; fixes #335; closes #336