[Branch]: differential_rotation

This is the feature branch that contains some diff rot code, I think it needs some love so I am opening this so people don't forget about it!!
It wont fall over pretty if you don't pass a Quantity something along the lines of "float object has no attribute to" where as "Input must be specified as Quantity" is much more useful, we should try and use the same error text code wide in SunPy.