Add basic tox setup (http://tox.testrun.org/)

Tox is a generic virtualenv management and test command line tool. It
allows testing with several Python interpreter versions in sandboxed
environments with automatic dependency installation.

Run as e.g.

TOX_LDFLAGS=-L/usr/lib/nvidia-current \
TOX_C_INCLUDE_PATH=/usr/lib/openmpi/include tox
Don't run tests in root directory

If we do we'll always pick up the pyop2 module from the working tree
and can't test the installed package (as we want to do with tox).
Use pip requirements.txt instead of bash script for additional dependencies