Adds automatic testing to `fusionbox.fabric_helpers`

`fabric_helpers.stage` will now check for a list of
test labels and apply `python manage.py test` to each.

`stage` will run no tests if nothing was passed at the
commandline.  Tests can be passed at the commandline
as a semicolon delimited string.

`deploy` will look in `env.tests` for a list of valid
test suites for Django to run.

Any test failures return a status code of 1 and will cause
fabric to immediately stop execution.
Merge pull request #45 from fusionbox/fabric_tests

Adds automatic testing to `fusionbox.fabric_helpers`