added in {name: db} object model to RS db users.js.  Replaced 'database'...

... with 'databases' in opts check.

Looks like the Rackspace API may have changed to require the database items to be objects of the form { name : database } instead of simply passing in the database name as a string:

http://docs.rackspace.com/cdb/api/v1.0/cdb-devguide/content/POST_createUser__version___accountId__instances__instanceId__users_user_management.html#POST_createUser__version___accountId__instances__instanceId__users_

There were also problems with the check for opts['database'] since all of the push operations were performed on opts['databases'].  I simply changed the check to look for databases instead; while this may break some compatibility with code already using 'database', it isn't working as is so will need an update regardless.  With that said, I also put together a commit that has functionality for both 'database' and 'databases' side by side... Let me know if ya'll would rather I submit that one.