Ticket #1498 - Conflict detection and resolution

This branch adds:
 - conflict detection code on Post and Page
 - a RevisionSelector view to compare N post revisions (it's used here with only 2 revisions, local and newer on server)

Notes:
 - wp.getPage doesn't reply with date_modified_gmt so currently, conflict detection can't work on Pages

TODO:
 - replace checkPostNotModifiedWithSuccess with a call to editPostWithSuccess