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

I mounted a VM with python 2.6, all worked except:
- argparse need to be installed, so i put it in the requirements files (do nothing in python 2.7)
- unitest is not so good in python 2.6, so i added unittest2 as a requirements, used for python 2.6
- i encountered some failures in tests with floats, so i passed them as strings and added a footnote in the fields doc about this

Then i ran the tests on travis on my own copy and all tests passed for 2.6 and 2.7.

I added badges to the top of the README files, with the pypi version and the build status on travis.You have to activate your repository on travis-ci.org to make the travis badge work (it will say "build: unknown" if not)