Cleanup unit-testing invocation

[adamhadani]

Currently we have the following:

1. a tox.ini file for running tests with the Tox test runner
2. a run_tests.py and run_tests_py3.sh wrapper scripts for running tests using nose, along with a skiptests.list file which seems to be empty / not used?
3. a .travis.yml configuration file for Travis CI which sets up dependencies / environment and invokes nosetests directly, along with associated requirements-py{2,3}.txt files

I would suggest we consolidate all of these to use tox so that a single tox.ini file contains all the definitions and dependencies required to run all unit-tests locally as well as in CI, so that travis CI's configuration file can be reduced to simply setting up tox and invoking it. We should be able to get rid of a few of the above files / definitions and make it clearer how to actually run unit-tests.

Along those lines, I would also consider adding running flake8 (Python linter / style checker) as part of running unit-tests on CI, this will help catch common issues early on (e.g unused imports/variables)

[adamhadani]

Fixed in #20