Merge pull request #83 from KennethNielsen/music_library_data_structures

First batch of music library data structures, version 2
stop py.test from searching tests in venv dirs
add .travis.yml for Travis-CI testing
Python 2.6 is not supported - remove from test envs
remove Travis' pypy target due to spurious failures

see #85
Re-integration merge of upstream master into music_library_data_structures_tests
Add test_data_structures.py to tests run by Travis-CI