Add new feature: git

This adds a new feature to manage remote Git repositories.

E.g.::

    from fabtoools import require
    require.git.working_copy(
        'https://github.com/disko/fabtools.git',
        path='/tmp/fabtools_git',
        branch='feature/git')

This clones the Github repo ``https://github.com/disko/fabtools.git`` into the directory ``/tmp/fabtools_git`` and checks out the branch ``feature/git``.  For all supported functions, see the included documentation.

Also the docs setup is enhanced by adding intersphinx support to enable direct links to the fabric documentation.