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.
Merge pull request #373 from caldwell/linedash

Add support for the lineDash API