Add tests

[ansman]

It's quite difficult to help write code for this if there aren't any tests.
The tests would be fairly simple to write as well.

Let me know if you want me to start on some tests.

[philfreo]

+1

[kurttheviking]

+1

[mikeric]

@ansman Definitely, that'd be a huge help if you've already given some thought into how and what should be tested.

[ansman]

I've started here: https://github.com/wrapp/rivets/tree/feature/tests

Feel free to comment on it.
I've added some rake and guard tasks to make my life a little bit easier when developing, they both require ruby though.