python-crossbar: new package

Signed-off-by: Mauro Condarelli <mc5686@mclink.it>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
[Thomas:
 - Remove dependency on having .py files, since building/testing with
   .pyc files only was successful.
 - Remove the "# runtime" comment for all dependencies, and only
   mention it once before all "select".
 - Add missing dependencies on: BR2_PACKAGE_PYTHON_PYCPARSER and
   BR2_PACKAGE_PYTHON_LMDB.
 - Use AGPLv3 for the license instead of AGPL3.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>