coverage and coveralls integration

sadly, I'm not proficient in tox-fu.
We need a way to let tests run (optionally, I'd guess) with coverage, that means:
- making sure coverage is installed (here python-coveralls has coverage as dependency, so we're good)
- instead of 

        python -m unittest tests

    run

         coverage run -m unittest tests

- run 

         coverage combine (not really needed, but useful if someone wants to do additional tests)

- run 

         coveralls

##### problems faced

tox and travis integration.... pydal uses tox as a way to keep travis.yml clean, but falls short in extensibility (probably due to my lack of tox-fu, from now on, referred as MLOTF). 
Keeping tox.ini this short means we can't have multiple commands in one-line (again, can be due to MLOTF)
Also, since tox creates a venv by default, just running "coveralls" or "coverage combine" in a after_success section on .travis.yml isn't a solution because tox changes dir after running the command. 
Ideally, on travis-ci we'd like to report coverage on any py27 run, while this PR is just a POC to make it run on py27-google. 
Again, ideally, we want a way to:
- run tests
- run tests against a specific backend
- run tests with coverage
and have options to do that. If tox is the designed/required tool, a way must be figured out.

##### considerations

running tests on pydal with tox is beautiful (if you're not on travis-ci, it's a good thing to have it), but without a fully-configured environment (especially the backends) is quite limited.... so I guess we just need to make a choice if we want a little bit of "more" clutter in .travis.yml and keep tox just to run 2.6 and 2.7 tests "smartly" (basically it's just a matter of tests discoveries) or "require" coverage to run tests for any 2.7, because that means basically changing the "commands" to

    commands =
        py26: {envpython} -m unittest2.__main__ tests
        py27: coverage run -m unittest tests     # <--
        pypy: {envpython} -m unittest tests 

or.... I don't know 'cause MLOTF . Feel free to suggest something.