Use tox to test on travis and add docs and flake8

Travis CI now also confirm that the docs build correctly and the flake8 runs
succesfully. Tox also has the advantage that it tests that beets installs
correctly.

Currently flake8 does not run succesfully so it is configured to not fail the
build on Travis.
Ignore generated man pages
Sphinx is required to build the package
We need sphinx before installing the tox environment
Add sphinx dependency to tox virtual environment
Use coverage only with py27
Merge pull request #643 from geigerzaehler/tox-flake8-travis

Use tox to test on travis and add docs and flake8