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!   
In `sunpy/database/serialize.py` where QueryEncoder gets defined you should probably change:
```python
values = o.min.value, o.max.value, o.unit
```
Then `json.dumps` will work fine.