Format rotated cell text

I must admit, it is ludicrous how long it has taken me to get to this. But here we are. 

This formats text appropriately for text rotated between 0 and 90 degrees. See below:

![prawn-table](https://cloud.githubusercontent.com/assets/1423683/4944165/148ee92c-65fb-11e4-9d5c-184898e414ec.jpg)

What remains:

* Angles between 0 and -90. Should other angles be considered?
* Right to left text. How should it behave?
* Skewed cells (for table headers.)
Seems like that line wasn't meant to be committed.