WP 3.6 Compatability

WP 3.6 launched with a major change to the revisions API, and a complete rewrite of the revision comparison interface, which made Post Forking's method of faking revisions to use the same interface much more difficult. So I implemented mergely.js, which keeps Post Forking's compare capability, and made adding a manual merge interface  trivial.

A couple of unit tests were broken, and a couple made invalid by WP 3.6's changes to revisions; I cleaned them up.

All this said, I'm new to Post Forking and may have missed a point or two—glad to take feedback and help resolve issues as needed.