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!   
@kaichogami this file should be removed once https://github.com/astropy/astropy/pull/2637 is accepted. The rest of the functions are already built-in in `quantities`