Levenshtein Refactoring :heart:

I refactored a bit the module which computes the Levenshtein Distance:

* Better clarity on some variable names
* Inline `min` function

On the other hand using an existing gem could make sense. [This one](https://github.com/dbalatero/levenshtein-ffi) uses FFI under the hood, thus I am not entirely sure if there might be some compatibility issue. It has been tested against:

```
MRI 1.9.2
MRI 1.9.3
MRI 2.0.0
Rubinius (1.9 mode)
```