Continuous integration

This is an attempt to:

* make the test run green over all supported Python versions: 2.6, 2.7, 3.2, 3.3, 3,4
* configure Travis CI to run the tests https://travis-ci.org/dimazest/nltk
* configure coveralls.io to monitor test coverage https://coveralls.io/r/dimazest/nltk
* installation of Numpy and friends using wheels to speed up installation

As a side effect I configured vagrant to have Travis like environment. Ideally, that virtualmachine should be enough to play with NLTK: all dependencies, parsers, provers, taggers should be there :).

So far it's a proof of concept. There are quite a lot messy commits. I wanted to squash them into bigger, but git is not the friendliest piece of software.

Things to be done:

* deploy hunpos model
* fix different set representation acros Python versions set([1, 2, 3]) versus {1, 2, 3}
* update readme
* decide where to host custom wheel files for numpy and the rest.