Supporting virtualenv developer workflow

Here's a minor problem I ran into. I usually develop with a virtualenv called "env" at the top level. Running py.test without arguments was recursing into env and running all the dependencies' tests.

Two minor overlapping changes I made to fix this:
1) run py.test (in tox and travis) as `py.test test` to explicitly run only the tests in the "test" directory
2) tell py.test and git to ignore the "env" directory

If you'd like I can re-do the pull request to eliminate (1) since it's basically superseded by (2).

It's no biggie if you don't want to worry about supporting a workflow you don't use i.e. reject this pull request (it won't hurt my feelings). If that's the case I'll try using buildout/mr.developer.

Unrelated: I couldn't figure out why your travis build was failing, and actually mine passed with the same code. Maybe you could try kicking off travis and the build will just pass?