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.
remove call to ConfigHandler::replacePath() when resolving autoload.ini entries.
replace is_readable() call with a silenced fopen(), which will check our include path for the file specified.

*BREAKAGE WARNING*
this means is that you must have a full path specified in autoload.ini.  "lib/blah.class.php" (to reference "/path/to/mywebapp/lib/blah.class.php") would no longer work - unless /path/to/mywebapp is in your include_path of course :)  This should only be a problem for user-provided autoload.ini entries, as the "stock" ones all reference the %AG_APP_DIR% constant to provide a full path.
Synced up changes from trunk, Merged revs 207:216