core: merge Mawuli's work for GSoC 2013.

Project title: "Zotonic Modules Repository Integration".

This work has resulted in a reengineered module command for the CLI: `zotonic modules`,
as well as a new ZMR admin client module: mod_zmm (not installed by default).

Fixes #672.

Notable comments from the merged commits:

Rename zotonic-installmodule to zotonic-modules.
Add more subcommands.
Add Mercurial remote url extractor:

  Given a local path to a Mercurial repository, return the `default` remote
  repository url.

  Fallback on another remote repository in the abscence of a `default`

Install directly from googlecode.
Add test fixtures.