Create library and example source & target apps
Add McLuhanAppDelegate

An app's UIApplicationDelegate will subclass McLuhanAppDelegate
and implement canOpenUrlScheme: to indicate that the app can
accept an incoming URL scheme.
Replace canOpenUrlScheme: with SchemeRegistry

Didn't like the reliance on the McLuhanResponder @protocol for
specifying an app's url scheme, as a subclass must remember to
implement its canOpenUrlScheme: method.

Much cleaner to simply have the subclass add its url scheme to the
registry.
Add means for posting message from source app to target app

As a part of this, the SourceApp now has an actions screen with
two actions: the first to send a test message, the second for
sending a user-entered message.
Make postToURLScheme: cleaner

Changed the name to callURLScheme: and it now uses a single
completion block in which the url and error are passed. Decided that
in most cases the success: block probably wouldn't be used.
Create library to house items common to source & target apps