Wip libradosstriper

Here is a first implementation of the blueprint presented at the last CDS concerning striping in ceph (https://wiki.ceph.com/Planning/Blueprints/Firefly/Object_striping_in_librados#section_4)

The code is by no way production ready (I'm writing the test part right only now) and should not be put in the firefly release, but I'd like to already have your comments, in particular on the new interfaces, and on as the packaging (separate library as discussed during the CDS).