Implement the RESTFul API of DeleteItem operation

Note that conditional delete still doesn't work because of
  - Bug #1301026, and
  - the following bug in Cassandra 2.0.6 (fixed in 2.0.7):
    https://issues.apache.org/jira/browse/CASSANDRA-6914

Implements: blueprint os-restful-api-delete-item
Change-Id: I9d6ff4f8c554e8a759de8d0f1641365092f246bf
Merge "Implement the RESTFul API of DeleteItem operation"