added notes about what dir AGAVI_INSTALLATION should point to if using svn
Revamped Controller::getGlobalModel to fix a fatal error if the model didnt exist.
As a bonus, added conditional checking so that we dont repeatidly hit the fs if we 
make multiple calls for the same model AND if the class is ultimately not found
we trigger autoload which will throw and AutoloadException if the class wasnt 
defined there either. So, you can define your global models in autoload.ini, which
as it turns out is pretty damned convenient.