Add Mercurial hook to post to ReviewBoard on push
The hook only allows to push if all commits have been approved in a

review requests, otherwise it rejects it and creates a review request

for the (new) commits. This is similar to "rbt post", but
1. Does not require the user to install RBTools locally.

2. Makes sure any changes pushed to the central server have been

   reviewed and approved.

3. Adds references to tickets/bugs/issues, which rbt post doesn't do.

4. Automatically finds the right review request to update if there

   are any new commits, based the commit ID and a date/author hash.

   This does not require the user to confirm anything, which rbt post

   (often) requires. This also allows the hook to recognize

   rebased/amended changesets, because the date/author hash

   is unchanged.

Created new review requests
Updated existing review requests
Tried to push not approved changesets
Pushed approved changesets
Checked matching of changesets with another date / author hash