Add methods for 'users' case
Changed the ´users´ methods to handle the user creation and authentication
Fix as request
Change the test messages
Add test for username isn't in the table, removed extra dependency
Fix in authenticateUser method the return when not found an auth user.
Merge pull request #6 from lamassu/users

Add methods for 'users' case
Clean expired pairing tokens
Do not parse data that's JSON already

It looks like using `json` type makes `pg` parse it as JSON
automatically.
Make pairing tokens use `timestamp` format and DB time
improved error message, look for multiple device records
tweaked for pg JSON field, cleanup