Added the user email address to the settings

This is the first half of the job.  This code
 - Adds a text input field to the Settings scene under the heading "Email
   address"
 - Adds the emailAddress property to the ConfigModel singleton, and
   saves/restores this to the user defaults storage
 - Does the update by calling the updateEmailAddress method of ConfigModel,
   which detects if there is a change, so the API call to the APN server will
   only happen when the email address is actually changed.
   - This means that when social media logins work down the road, they can call
     the updateEmailAddress method and no additional code will be necessary
Code complete for creating the REST callback to the server to update email addresses

The address is stubbed out right now since Karlon hasn't told me that his
server is ready, and I don't want to accidentally bork the APN demo by feeding
input to his server that it's not ready to handle.  It'll be a one line change
to enable it when he's ready, and we can demo everything regardless (they won't
know that the data isn't actually making it to his server...)
Merge pull request #40 from agricocb/master

Added user email text field in the Settings scene