Add handling for Mobile worker registration via SMS

Refer to: http://manage.dimagi.com/default.asp?149567#842272

@gcapalbo I have tested that changing the settings sets this value for a domain.  Is there an easy way to test the actual registration via SMS?  I also require both a username and password, but I am not sure that is actually the behavior we want.  What do you think?
likewise - do we have to worry about this number already being in the system somewhere here?