Add a table for storing verification codes

Because we're gonna need to remember what the user
is supposed to respond with.
Add nexmo submodule. Abstract nexmo stuff.

Add a callback for incoming SMS messages. Add an exec script
to change the settings strings stored in the database.

The SMSLib library abstracts sending and receiving sms messages
to a static function level. This is because the constructor for the
nexmo object requires passing around keys and such, so I sort of
made it into a singleton (not really) in that it instantiates and
then hands you objects to use.