Clip long font tile labels

Previously font tile labels that didn't fit on the tile would bleed right to the edge of the tile. This explicitly clip long labels with an ellipsis:

![...](http://f.cl.ly/items/423Q041t460H2v1Z3q3a/Screen%20Shot%202013-09-13%20at%2011.28.59%20AM.png "...")

Addresses #150

Assigning to @larz0