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!   
What's the advantage of require an `Angle` instead of a simple `Quantity` with angle units? I think that for this function I would do:
```python
try:
    latitude = Angle(latitude)
except UnitsError:
    raise ....
```
@Cadair, what do you think?