add Shot model to admin
fix reverse lookup for node views
minor pep8 style fixes
notification in device detail template if there is no latest shot set
REST API for setting latest shot

Using HTTP POST to /<device>/latest/ with data shot=shot_number, a new
shot object will be created, populated and set as latest shot for device.

Population of the data tree in sql is now done aynchronously.

hint, can use curl, for example
curl -X POST --data "shot=999" http://localhost:8000/data/h1/latest/