Switched the ConfigModel to use the singleton pattern and made a minor enhancement to Install_GMapsAPI.sh

Made the ConfigModel object a singleton, rather than passing it around between the UIViewControlers.  That looks like it would have been do-able, but I probably should have just made it a singleton to begin with, so that I don't have to wire it expressly to the UINavController, and then send it in to each UIViewController when they are put on top of the view stack.

Also, in the Install_GMapsAPI.sh script, I just added the --key and --unkey command line options to the script so that it is more convenient to add or remove the Google Maps API key from AppDelegate.m, which one might want to do before a pull/merge that includes changes to AppDelegate.m.  It is still called by the build stages automatically, and will only add the key when called without options if it is also installing the GoogleMaps.framework; this might be desirable to change, but for now I'm keeping it completely identical in behavior to what it was before.  The --key option, though will force the updating, but only from the unkeyed state (as AppDelegate.m is in the state committed to the repository), and it will do it from the cached version (which is stored along side where ever it finds the .zip file for the framework.)  The --unkey flag should leave it in the state that it should be in before commiting any changes to AppDelegate.m to the repository.