Test cases as documentation

Hey, i've knocked together a couple of test cases for your *users* role. I wired up Travis CI [to run them](https://travis-ci.org/CraigJPerry/ansible-users) too - my first time doing that; another TODO checked off the list!

These might be useful to other users of your role as documentation or to help raise confidence the role works[^1]. This role currently has zero ratings on Galaxy, which is a pain since it works perfectly fine by my testing!

Anyway, hope it's useful. In the future i'll fire in a few roles to Galaxy myself. Hopefully i can encourage providing tests on shared roles.

> "You can judge code by its tests"
> *Probably someone said this, sometime, maybe*

It'd help me when selecting which role to choose!

[^1]: at least for the given test cases, in the latest ansible available from PyPI, on the Ubuntu 12.04 LTS OS used by Travis CI