examples: add a librados/hello_world program

This is a simple program with lots of explanatory comments people
can use as a model for using librados.

Signed-off-by: Greg Farnum <greg@inktank.com>
Merge pull request #508 from ceph/wip-5905

examples: add a librados/hello_world program

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Loic Dachary <loic@dachary.org>
Reviewed-by: Dan Mick <dan.mick@inktank.com>