add pytest markers and refactor testenvs to separate unit/func/integ/accept

[mattsb42-aws]

Problem

Currently, all the tests run as a big batch, with the integration tests skipped unless a specific environment variable is found.

This is a bit clunky and makes it difficult to tell in Travis whether integration tests were run or skipped.

Solution

1. Add pytest markers to all functional, integration, and acceptance (xcompat) tests.
   • https://docs.pytest.org/en/latest/example/markers.html
2. Update tox to have separate testenvs for unt/func/integ/accept tests.
   • http://tox.readthedocs.io/en/latest/config.html#generating-environments-conditional-settings
3. Update Travis CI configuration to run the new tox testenvs.

[mattsb42-aws]

Addressed in #41