Implement initial Python bindings for Open vSwitch database.

These initial bindings pass a few hundred of the corresponding tests
for C implementations of various bits of the Open vSwitch library API.
The poorest part of them is actually the Python IDL interface in
ovs.db.idl, which has not received enough attention yet.  It appears
to work, but it doesn't yet support writes (transactions) and it is
difficult to use.  I hope to improve it as it becomes clear what
semantics Python applications actually want from an IDL.