implemented update and register device operations
check if the device belongs to the user
fixed device unit testing
slight modifications to work with new version of stacksync-commons
'Object' naming renamed to 'Item'. Removed some useless attributes from
the models. IDs are now created by the server.
first version of sharing protocol
Created new exceptions and new exception handling.
added a new field to Workspace to know whether it is a shared workspace
or not.
Sharing feature implemented. Created storage manager to interact with
OpenStack Swift.