Provide explicit 'execute_from_command_line' variants which take no args...

Created by: olibrook

On issue #18 (closed) it looked like people were in favour of setting up different manage.py versions (eg. manage_remote.py) in order to save typing --sandbox=remote --settings=myapp.settings.remote every time. This makes sense to me, and we have started doing it on our projects.

The attached changes provide entry points for management commands which use remote, local and test sandboxes without the optional sandbox argument. This makes it a bit easier to write these manage.py scripts without knowing about djangae's context managers and simpler to use features like setuptools entry points, if that is how you roll.

The existing execute_from_command_line and sandbox argument are unaffected.