Add support for testing with tox (http://tox.testrun.org/)

This will allow easy testing across multiple Python versions - e.g.: 2.6, 2.7, and 3.3.

Basically, you just run `tox`. For more info on tox, see http://tox.testrun.org/

The motivation for this is I want to tweak `rxjson` to be compatible with Python 3 as this is  needed for projects that depend on `rxjson` to work with Python 3 (e.g.: [cornice](https://github.com/mozilla-services/cornice)).

Sample tox output:

```
marca@marca-mac:~/dev/git-repos/rxjson$ tox
...
  py26: commands succeeded
  py27: commands succeeded
ERROR:   py32: commands failed
ERROR:   py33: commands failed
```