Add run_tests script, run automatically in Travis

Add a test runner script which runs any units tests using nosetests
(there currently aren't any) and will eventually run PEP8 tests.

This will run on Travis CI after visiting https://travis-ci.org/profile and
enabling it for the repo.

In Travis CI, we need to use system site packages for numpy and scipy,
see:
http://danielnouri.org/notes/2012/11/23/use-apt-get-to-install-python-dependencies-for-travis-ci/

For Travis to try to install the right packages (not numpy/scipy),
create two requirements files: `requirements/_common.txt` and
`requirements/_numpy_and_scipy.txt`.

As a developer, you can install `./requirements.txt`. As Travis, you can
install `./requirements_for_travis.txt`

In the `.travis.yml` file we also use the environment variables for
ATLAS, BLAS etc as suggested by Sam in the README:

```
export LAPACK=/usr/lib/liblapack.so
export ATLAS=/usr/lib/libatlas.so
export BLAS=/usr/lib/libblas.so
```