Use Damerau-Levenshtein to improve british spellings

Summary: Price transpositions very cheaply. We might need to edit these weights a bit, but this covers the two previous cases ("test", "alnd") and gets them right.

Test Plan: Unit tests, various `arc x` tests.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D5034