Add support for the lineDash API.

Described here:
http://www.w3.org/TR/2dcontext/#dom-context-2d-setlinedash

The test images match on Chrome, but not on Firefox. It looks like
Firefox resets the lineDash to [] when given invalid parameters. My
reading of the spec agrees with Chrome.

-David
I just tried with node-canvas 1.1.5 on Mac OS X 10.9.4 (Xcode 5.1.1) and it compiles fine for me. Those defines should only kick in if isnan() isn't defined (IE Windows), but isnan() should be defined in Mac OS X's math.h (and is on my machine). Is there a reason your math.h might not be included, or might be different from the standard one?