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

On issue #18 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.