Fix unit tests

Looks like the reason of failing tests I was reporting in #34 is `levenshteinDistanceWithString...` method. I didn't understand what exactly wrong with the method. But it's very important to have unit tests working so I think we can remove that method.  Especially because existent unit tests already confirm the bug I've noticed in my production code and have fixed here #41

![2014-10-05_2253](https://cloud.githubusercontent.com/assets/217368/4519851/c08a1090-4ccd-11e4-8d80-6681c8f050bb.png)

Is is expected that 2 unit tests are failing when you are running from this branch (on Travis CI as well). Please accept #41 as well to make everything passing :)