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.
Merge pull request #135 from coldwd/patch-1

adding PIL pre-req documentation
Merge branch 'master' of github.com:benjaoming/django-wiki
JSON view can return HttpResponseRedirect
fix imporerror for HttpResponseRedirectBase
Do not use HttpResponseRedirectBase anyways, just check status_code
More clear PIL instructions.
Merge remote-tracking branch 'origin/master'

Conflicts:
	README.md