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)

Consider using alloca or malloc instead of new. You can also set a threshold to prefer alloca based on buffer size.