Update a Photo from the API

[kappuccino]

We need to be able to update a Photo using the API.

After a quick talk with @toorop we decide to do the following (at least for the biginning)

A full (struct) Photo is sent to the API with an updated version of some fields (Name, Description, Tags ...). Some fields should not be updated, like viewCount for instance — In order to deal with this, the server will only update a predefined list of field, excluding dangerous update.