House cleaning: README correction, HTTP status code 204, return $this, and returnbody

This PR is to address issues #11 (CLIENTS-85), #27, #72 (CLIENTS-87) and #80 (CLIENTS-52) for Riak 1.4.x client. These are fairly small changes to do a little bit of house cleaning on the issues list. I corrected the README by adding a missing 'new' keyword, added the ability to override the returnbody parameter on store, added $this to the return for addIndex to make it consistent with the docblock, and added support for the Riak 204 status code for PUTs.