#NSString-Demerau-Levenshtein
Fork from https://github.com/JanX2/NSString-DamerauLevenshtein.
From the Wikipedia article:
The Damerau–Levenshtein distance between two words is the minimum number of operations (consisting of insertions, deletions or substitutions of a single character, or transposition of two adjacent characters) required to change one word into the other.