Ticket #1498 - Conflict detection and resolution + use wp.editPost instead of metaWeblog.editPost

This branch adds:
 - conflict detection code on Posts
 - a RevisionSelector view to compare N post revisions (it's used here with the only 2 conflicted revisions)
 - switch calls to metaWeblog.editPost with wp.editPost and metaWeblog.newPost with wp.newPost
 - date_modified_gmt in BasePost data model

Known issues:
- A user can edit tags/categories but he can't remove all tags (or categories) from a Post.
- wp.getPage doesn't reply with date_modified_gmt so currently RevisionSelector is not plugged in PagesViewController.