[makefile] Cleanup, add non-testing default "help" target

This allows to test with different Python versions, e.g.

  make PYTHON=python3
  make PYTHON=python2.7

Also, avoid running tests by default and show usage info instead.
[tests] Cleanup project structure, add tests from gPodder

 - 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"
add encoding info to test_podcastparser.py

Tests fail on Python 2.x otherwise
Remove coveralls from requirements-test.txt

This is not a dependenc for running the tests, its only used within Travis-CI