Add json fixture for devices
Modify existing json fixtures slightly.
Remove xml fixture that is not being used.
Add manager methods to get objects by user.

The following model's managers have a getEditable method that takes a
PortcullisUser and returns the objects that user is allowed to edit:

Key, ScalingFunction, Device, DataStream, and PortcullisUser.
Add unit tests for models getEditable methods.
Change read_source to use objects.getEditable method.
Add South to portcullis, and allow zoom_start/end to be null.
Keep track of zoom level in shared view.
Fix setup so it uses south.
Merge branch 'development' of github.com:visgence/portcullis into development
Add more keys to fixture for different scenarios
Add get_reabable_by_key to DataStream manager
Add tests for DataStream get_readable_by_key
Merge branch 'development' of github.com:visgence/portcullis into development