Add tox.ini for tox (http://tox.testrun.org/)

Adds support for testing against multiple Python versions with [tox](https://testrun.org/)

## Example:

```
marca@marca-mac2:~/dev/git-repos/pytest-sugar (master●)
$ tox
...
  py26: commands succeeded
  py27: commands succeeded
  py33: commands succeeded
  pypy: commands succeeded
  congratulations :)
```