Use string when passing floats to Redis (in tests)

It avoids problems with float precision.
Also add footnotes in fields documentation about this
Add argparse and unittest2 requirements to work with python 2.6
Add travis conf for current dev branch only for now
Remove branch whitelist in travis conf, ready for master
Add badges for pypi and travis on README.rst
Add PyPI and travais badges in the doc index file
Correct uses of the old get_pk in the documentation
Merge pull request #80 from twidi/python2.6+travis

Add conf for travis (+badges), add support for python 2.6, and precisions about floats