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