Add failing tests for issue #15
Revert Levenshtein maximum distance

It's not immediately apparent whether this kind of short-circuiting is
actually possible. As it's not been part of a release, let's revert it
so that master contains working code.

* Reverts af21890
* Fixes #15 (or at least defers it)
Avoid iterate full word when the diagonal value is greater or equal to max_distance
Fix: Diagonal index was not correctly calculated in some special cases when levenshtein matrix was not square