pybind: convert librbd bindings to Cython

pybind: convert librbd bindings to Cython

ctypes is fundamentally incapable of providing safe callback support
(exceptions during callbacks are not handled properly). To fix this, and also
gain more flexibility, rewrite/port the librbd bindings to Cython instead.
Other Python bindings are expected to migrate to Cython in the future.

Reference: http://tracker.ceph.com/issues/13115

To support this, this change also:
- Introduces a Makefile for pybind which calls setup.py
- Updates the installation files for packages
- Adds a hack to admin/build-doc to build a dummy librbd, avoiding the need to
  build the entire client library just to generate documentation (Sphinx needs
  to be able to import rbd.so, which depends on librbd.so symbols).

With contributions by Josh Durgin <jdurgin@redhat.com>

Signed-off-by: Hector Martin <marcan@marcan.st>