Cornice with support for Python 3.3

Thanks to @msabramo' s intervention, simplejson is now in Python 3.3 and -therefore- support for Cornice in this release is pretty simple.
 
I have run all tests in 2.6, 2.7 and 3.3 and it passed all of them (although in 3.3 there is an incompatibility problem in rxjson -easily solved by running 2to3 on rxjson/Rx.py-).
I would like you to consider my pull request. In case you consider some specific test is missing and you would like me to add some extra tests, just let me know. I will be more than happy to help.