Fixes and cleanups

Add more tests from `gpodder-core`, remove Travis-CI related trash (the requirements list is small enough to put into `.travis.yml` direcly, and putting the CI status indicators - while probably very hip these days - into the README file is not really useful, *especially* if we're going to use the `README` file as long description in the `setup.py`.

As we use `nose` for running our tests, we don't have to use the Java-style vintage `unittest` module and request `doctest`s manually, but actually all of that can be done using nose and `--with-doctest`.

If you want to have the Travis CI icons/status indicators somewhere, put it on a wiki page, e.g. on wiki.gpodder.org, or we can also add a special podcastparser page to the gpodder.org webpage. Or on readthedocs?