rbt land supports Git, this change adds support for Mercurial as well.

rbt land works on bookmarks in mercurial. 

Mercurial supports merging and deleting bookmarks but not pushing upstream because

bookmarks are usually not shared with the upstream, so it is left to the user to 

decide what they push.

Ran all unit tests 

Added unit tests for the new mercurial client functions.

Tested some rbt land commands on a Mercurial repository including --local,

not local, --squash, and --no-squash.