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!   
If diff is a `datetime.timedelta` object then we could change that by: 
```python
diff_s = diff.total_seconds() * u.second
```