Refactored the python bindings into multiple modules.

The existing module has been split into two separate ones:

- mesos.interface - This contains the stub implementations for the
                    Executor/Scheduler as well as the protobufs.
- mesos.native - The old _mesos module.

There is also a base metapackage `mesos` that allows a potential `pip
install mesos` to correctly install everything required. While
mesos.interface can now be uploaded to the cheeseshop, mesos.native
has not changed and will need some more work first.

Review: https://reviews.apache.org/r/23224