Fix for vertical centering of text when using different fonts

This uses glyphs to measure the visual height of the font instead of the computed height.

![helvetica](https://cloud.githubusercontent.com/assets/226553/3191018/c1a24536-ecd5-11e3-87c0-d5c56d1427c4.jpeg)
![whitney](https://cloud.githubusercontent.com/assets/226553/3191019/c1a25daa-ecd5-11e3-832c-9a936554d59d.jpeg)