User can specify GlslProg for use when rendering TextureFont.

Pass as `TextureFont::DrawOptions().glslProg( shader )`

Sample result:
![screen shot 2014-12-23 at 5 40 26 pm](https://cloud.githubusercontent.com/assets/81553/5543991/77f92eb0-8acb-11e4-8380-26ec393e8b83.png)

Also includes the bugfixes from #644