[tests] Cleanup project structure, add tests from gPodder

 - Remove travis-related stuff (requirements-test.txt, README.md)
 - Rename the README file to make "sdist" happy
 - As we use nose, no need to use "unittest" and "doctest" modules
 - Import test cases from the "gpodder-core" project
 - Merge gpodder-core's test runner with existing tests
 - Rename the test script to "test_podcastparser.py"