id attribute feature + many goodies: version 0.1 final

* id attribute feature as per #13 + docs and tests
* increased test coverage
* added a working .travis.yml for continuous integration testing with travis and coveralls
* renamed CONTRIBUTORS.txt with AUTHORS, which is more common in python packages
* added a LICENSE file, which was missing
* added travis and coveralls badges to README

I added a permissive common license to the repo, because it should be there if we want to package it and upload it to pypi. I added the MIT license, but if you don't like it please change it to what you would prefer.

Also, the badges reference my repository, so after you merge you should add your repo to travis-ci.org and coveralls.io and change the badges.

after merging this we can change the version from "beta" to "final", tag it here on github and do #12

Federico