feat: delete block endpoint

Blocks can now be deleted, if they have no references. A status
code of 204 is then returned.
A Constraint Error is raised by the metadata driver if there
are references, and 412 is returned as status code.

Endpoint:
    DELETE /v1.0/vaults/{vault}/blocks/{blockid}
Thanks for the comment!
Its being done here: https://github.com/rackerlabs/deuce/pull/125/files#diff-ab242dc412d90b6e559f62a8750ea2a0L118