gitignore: Ignore files generated by python-coverage.
requirements: Add python-coverage as requirement for code coverage.

We should get our code covered by as many tests/execution parts as possible
to ensure that we deliver a good experience to our users and to make sure
that we don't have any lingering bugs out there.
travis: Hook `coverage` with `nosetests`.

We limit the package that we are interested in to only `coursera` (maybe to
be changed to `coursera_dl` after a potential merge with other projects?).

Given that the code tested is *not* 100% the same as the code executed
when really downloading things (due to stubs etc.), we need to check what
options we have to cover real execution as well.

Note that this still is not sufficient to get the results in
https://coveralls.io/r/coursera-dl/coursera as we need to communicate the
results to them.
README: Make coverwalls happy with the badge included in the README.

Otherwise, I get a huge banner telling me that I should include this.