adding PIL pre-req documentation

Without libjpeg PIL won't be able to install with jpeg support and users will see IOErrors on their wikis.  Highlighting this in a "prerequisites" section on the README.

Documentation now has a little bit of background as well as installation instructions for Linux/OS X.
changing apt-get to use python-imaging

Per @benjaoming.
JSON view can return HttpResponseRedirect
fix imporerror for HttpResponseRedirectBase
Do not use HttpResponseRedirectBase anyways, just check status_code