Feature/glyph padding

Added creation of signed distance fields for the glyphs along with patrick's glyph cache changes with padding and glyph render callback
![font](https://cloud.githubusercontent.com/assets/4699905/3709051/315bd118-144e-11e4-8c01-57662dac042c.png)

Seems like a lot of work just so you can do [x][y]. Think about allocating one buffer and stepping y by width instead of 1.