Enable rotation of CategoryAxis tick labels by angle.

Right now only 0 (horizontal), 90 (right), and -90 (left) are supported.
This feature is not well supported and will only work for short strings.

Related to #504

Image of new quicktest: 
![image](https://cloud.githubusercontent.com/assets/1400023/4468738/8ee1ae0e-4903-11e4-9cba-948505f3b06f.png)

What's the reasoning behind this?