Add pytest integration

[toolness]

I just noticed this note to users in the nose docs:

Nose has been in maintenance mode for the past several years and will likely cease without a new person/team to take over maintainership. New projects should consider using Nose2, py.test, or just plain unittest/unittest2.

So we might want to migrate to py.test since it seems to have the closest feature parity with nose.

To do:

• Figure out why py.test finds 188 tests, while nosetest finds 191. 😟
  • It looks like nose is running our db_test_request_context helper function three times! Lame.
• Remove nosetest package via our docker-quick/Dockerfile and make sure everything still works.
• Change readme to mention py.test instead of nosetests or nosetests.sh.
• Change .travis.yml to run py.test instead of nosetests.sh.

[toolness]

Here are logs of running py.test and nosetests for reference:

pytest.txt
nosetests.txt