Initial check-in of nemo-rabbitvcs modules.

This is a port of the rabbitvcs-nautilus module to nemo. It is a
straight name-swap from nautilus to git, with two notable changes.

1. RabbitVCS expects to find the environment variable
NAUTILUS_PYTHON_REQUIRE_GTK3 set if it should load GTK3. The name of
this variable cannot be changed as a result.

2. make_menu_item() constructs a proper Nemo menu item in the extension
code. The original extension called back into the rabbitvcs-core code,
but there was no reason for it to do so.
nemo-rabbitvcs Debian packaging.

Packaging configuration for nemo-rabbitvcs. The dependencies have been
updated to depend on python-gi >= 3, since the code only uses
introspection and this is a package independent of rabbitvcs now.

AUTHORS and MAINTAINERS files updated to reflect distribution of
responsibilities.
Updated nemo-extensions README file with new status.
Changed maintainer field to be Clement Lefebvre <root@linuxmint.com>.
Removed the use of get_data() and set_data() as they are now deprecated.
Merge pull request #12 from wrouesnel/rabbitvcs

nemo-rabbitvcs