Adding configuration for tox

Tox will need to be installed on the build servers (pip install tox). This provides separate venvs for every test job.

Jobs defined: py26, py27, py33, pep8, cover

Modified Makefile to use tox:

Replaced 'make unit' with 'tox -e py27'
Replaced 'make coverage' with 'tox -e cover'
Replaced 'make lint' with 'tox -e pep8' (the pep8/pyflakes part of the lint script)
The cover job, will produce a _coverage_html directory, and coverage.xml file (which can be imported into jenkins with the Cobertura plugin)

The py27 job will output xunit to test-report.xml