Integration of astropy.units in sunpy codebase

Attempt to integrate the astropy.units into sunpy. Every function that were taking or giving a quantity now must have astropy quantity associated with them. Please tell me if I have made mistakes anywhere, or if it could be done in better way, I will make changes! 
Thank you!   
``Angle`` is a subclass of ``Quantity`` that adds convenience things like formatting angles to strings (e.g. for coordinates) and also adds properties like ``degree``, ``radian`` for fast access to the values.