new: use ``requests`` instead of ``urllib2``.

This avoids much of hassle with gzip encoding, multipart,
authentification, url encoding... which this module should avoid
managing because requests does it better and is widely supported and
tested.
This allows wikitools to concentrate on its main task.

Please don't accept this code for now: as there are no tests, I didn't check enough yet the multipart and encoding parts... This is a quick overview and I wanted to know if you are open to this type of changes.

I would have also other changes to propose (I wanted to apply some general PEP8 considerations), would you be open to such pull-request ?

Do you know if third party software are currently relying on your code ? I just want to get in hand some code to test that nothing is broken.

Thank you very much for your work on wiki-tools !