Automatic testing with Travis CI

Run our `nosetests` and `mpiexec` tests with Travis CI on Python 2.7 and Python 3.3.

The `exercise_odin.py` tests for `odin.py` aren't included at the moment because of a weird import error on Travis.  We should change `odin.py` to be runnable by `nose` anyway, so I'm not worried about this.  `exercise_odin.py` *does* still run on *my* machine.