[api][rackspace]Added ability to add multiple users to databases on same instance

I wanted to throw this one up to get some feedback. Currently the api will work as it currently does when receiving a user to add to particular databases. I am also currently working on the test to ensure that my implementation for adding multiple users to the same instance works.

The question I do want to propose is how exactly to pass the instance for this API. Currently the assumption is made that if there are multiple users, both contain the same instance that they are being added to. This is to ensure that one request is being made following the rackspace [api][api]. 

I am also investigating how the databases are added to the user objects. According to the previous link, the rackspace API dictates that the databases are passed as an object with a name attribute. I do not believe that is how it is done currently but I cannot test against a real case to confirm it. 

Let me know what you guys think and I'll make the changes that are required. Thanks.

[api]: http://docs.rackspace.com/cdb/api/v1.0/cdb-devguide/content/POST_createUser__version___accountId__instances__instanceId__users_.html