Python3

I started from dominicrodger's PR, and did:
- rebase against master
- added additional required fixes for python3
- updated tox. added .travis.yml, and coveralls


Note: I was not able to use runtests.py (it is not used by tox/.travis.yml) - perhaps that one should be removed? (in favour of the setup.py test).

Note:  seems some tests are missing, in particular, these methods are not covered at all:
- value_to_string
- formfield

and a uuid version1 should be created as well.