Pdpinch/documentation updates

All the docs, including installation and course author documentation, are in the ReadMe now. 
That would be a nice feature (to log or notify at startup), but it would be nearly impossible to implement since you would have to check all the courses for xblock tags and see if you have them installed or not.  I think you may be conflating xblocks with django apps as well.  XBlocks aren't supposed to be django apps, so they shouldn't even be listed in any django apps list.  They are discovered at runtime by using entry_points, and thus should be INSTALLED_APPS or OPTIONAL_APPS.