Add python-bson to dependencies

This dependency was missing when building with zeromq as the transport
for RFProtocol.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Ensure that git branches are always checkout out

We would previously only change branches when updating.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Improve usage description

Signed-off-by: Joe Stringer <joe@wand.net.nz>