Allow for editing document's privacy

- Add PUT to allowed verbs in document resource
- Merge existing raw data with updated data to avoid overwriting fields
 in a map.
- Use prepared query since I couldn't find how to update a list at a
 given index using hayt (waiting to hear from Max Penet about this)

Fixes #544 

Viewing: 
![image](https://cloud.githubusercontent.com/assets/2522010/5458822/bb6589c6-8555-11e4-806d-fe8f88326698.png)

Editing:
![image](https://cloud.githubusercontent.com/assets/2522010/5458810/a60cbda6-8555-11e4-910d-26755868a824.png)


Happy to put it in a separate PR. Would you raise a ticket for it as an enhancement?