Add support for patternUnits attribute. Fix #1535

This fixes #1535

The `patternUnits` attribute defines how a pattern's coordinate system is defined (x, y, width, height). This is particularly important when using repeatable patterns in SVG. This commit adds this attribute to the lists of known properties.

I used #1487 as a reference point, please let me know if this needs to be added anywhere else.

![result](https://cloud.githubusercontent.com/assets/590904/3005355/f2f46d16-dddb-11e3-9829-174edf4c71bd.jpg)