Implement 'devel' subcommand

Implement 'devel' subcommand

This command is to support another development model where packaging
files are maintained in a separate branch and source code changes in
separate development branch(es). The devel command is designed for
managing these development branches.

The subcommand provides the following actions:
1. gbs devel start: initialize development branch
2. gbs devel export: export patches from development branch back onto
                     the packaging branch
3. gbs devel drop: delete the development branch
4. gbs devel switch: switch between packaging and development branches
5. gbs devel convert: convert a package from the "old single branch"
                      development model to the new model

Change-Id: I2eec2b2ed73f50e03a9f7a2e5f6d24fc765c3a5b
Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>