Bug 1039894 - Add bzpost and firefoxtree to mach mercurial-setup; r=smacleod

bzpost is an efficiency win for everyone.

firefox tree is mostly useful for people doing head/bookmark-based
development.

Both extensions can yield massive productivity wins. They should be
advertised.

DONTBUILD (NPOTB) on a CLOSED TREE

--HG--
extra : rebase_source : ab72d3afd6ab244bfdbdb2dda5e0b45c9d8f782e
extra : amend_source : 91ec7523a4631c7519efd3b6cd425cff00bd0569