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!   
Same than above... by why using `Angle` instead of `Longitude`?  maybe it can be negative?  `Longitude` wraps in a 0-360 range, so probably it's what we need here too.