Use tox to test on travis and add docs and flake8

[geigerzaehler]

Based on the suggestions in #631.

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. This branch is intended to make flake8 pass. But we still have to agree on style guide.

[sampsyo]

Shouldn't this include py26 and pypy so that just typing tox runs the same five configurations as Travis?

[geigerzaehler]

I thought of this as more of local pre-commit test to be run by the user. I don’t have 2.6 and pypy on my system so I would get errors. Even if they were installed, it would nearly triple the testing time to almost no benefit because incompatibility errors are rare.

[sampsyo]

That's a good point. I like to have all three environments because I mess things up for 2.6 occasionally and it's nice to see exactly what Travis will run. Either way is of course cool with me since the -e flag there for overriding the default set.

[sampsyo]

Also, FWIW: I've been interested in trying detox to speed this up on my machine.

[sampsyo]

I love it! ✨ I didn't know Travis gave you access to all the Python version even when the Travis environment was fixed (2.7 here). This is great since it gives tox control over what should run so we can test it locally. Yay!

I'm so excited about having this (including the docs build on Travis) that I'll merge immediately.