levenshtein distance on strings

A simple implementation of an [edit distance between strings](http://en.wikipedia.org/wiki/Levenshtein_distance). If it's not already somewhere in batteries, I think it might be useful. A possible extension would be to use a [VP-tree](http://en.wikipedia.org/wiki/Vantage-point_tree) (for instance) to retrieve, in a big set of strings, the n ones which are the closest to a given query string.