Extend Pithos backend functionality in order to
be able to set the state of the snapshot.
This commit introduces the appropriate modifications
proposed in #231. More specifically:
* Changes 'available' flag (denoting the existence of the object mapfile
  in the backend) to integer.
* The 'available' field can be assigned with the following values:
  -1 (MAP_ERROR), 0 (MAP_UNAVAILABLE) and 1 (MAP_AVAILABLE)
* Provides a respective database migration script.
* Adds 'update_object_status' method in PithosBackend that is used in
  order to update the state of a Pithos object.
* Returns 400 (BAD REQUEST) response status when one tries to download a
  broken snapshot.