Added python wrapper to rados_cluster_stat

The new get_cluster_stats() method on the rados.Rados object calls
the rados_cluster_stat() function in the librados library.

Signed-off-by: Christopher Glass <christopher.glass@canonical.com>
Merge pull request #31 from chrisglass/expose_cluster_stats_to_python

Added python wrapper to rados_cluster_stat