Tox support

I added [tox](http://codespeak.net/tox/) support which can be used to test the library with multiple Python versions. Just run ```tox``` in the project main directory.

My current results (I don't have the 04d8:fa2e USB device):
```
  py25: commands succeeded
  py26: commands succeeded
  py27: commands succeeded
  py31: commands succeeded
  py32: commands succeeded
```