Levenshtein distance algorithm for general array.

It provides:

• Levenshtein alrogithm by Wagner-Fischer algorithm: http://en.wikipedia.org/wiki/Wagner%E2%80%93Fischer_algorithm
• Correctness is tested comparing against a naive but more mathematical algorithm.
• Upperbound parameter to stop the comparision once the distance is found bigger than it.
• Memoisation cache to avoid repeating distance calculations.
• Functor to abstact the implementation of the array and cache.
• Ready-to-use String and StringWithHashtbl modules are provided.