Add Python support library for remote helpers

Add Python support library for remote helpers

This patch introduces parts of a Python package called
"git_remote_helpers" containing the building blocks for
remote helpers written in Python.

No actual remote helpers are part of this patch, this patch only
includes the common basics needed to start writing such helpers.

The patch includes the necessary Makefile additions to build and
install the git_remote_helpers Python package along with the rest of
Git.

This patch is based on Johan Herland's git_remote_cvs patch and
has been improved by the following contributions:
- David Aguilar: Lots of Python coding style fixes
- David Aguilar: DESTDIR support in Makefile

Cc: David Aguilar <davvid@gmail.com>
Cc: Johan Herland <johan@herland.net>
Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>