Add support for N-gram distance

[larsga]

From [email protected] on March 08, 2012 11:31:27

This is not the same as Q-gram difference (even if Q-grams and N-grams are the same thing). Described in this paper: http://www.cs.ualberta.ca/~kondrak/papers/spire05.pdf Appears to be O(n^2), and so QGrams seems more important.

Original issue: http://code.google.com/p/duke/issues/detail?id=71