Add paramiko wrapper

Created by: player1537

These commits include everything needed to get the test cases working on Travis.

To do this, I needed to create a RemoteWrapper (in django_remote_submission/remote.py) and simplify the code to use that wrapper instead of calling Paramiko directly. This meant that in my tests, I could create a LocalWrapper (in tests/test_tasks.py) and use that in place of the RemoteWrapper.

On Travis, the code will check to see if it is running on continuous integration, and if so, it will only run the local tests (using LocalWrapper). On a regular host, like my mac, it will run both the local and remote tests, provided there are environment variables in the .env file.