Fontstash implementation

Move from FTGL font rendering to this [fontstash implementation](https://github.com/tangrams/fontstash-es):
+ removed FTGL library
+ gpu text buffering on all labels of the tile once when tile is loaded
+ outlines rendering using signed distance field

Visible differences from implementation, glmFeatureLabelLine :

![capture d ecran 2014-10-17 a 14 19 14](https://cloud.githubusercontent.com/assets/7061573/4738799/a38f9642-59ff-11e4-98fd-b6999a3f5f12.png)
![capture d ecran 2014-10-17 a 14 19 46](https://cloud.githubusercontent.com/assets/7061573/4738800/a396b1a2-59ff-11e4-9f43-3d0f499f922a.png)