Support HTTP 201 Semantics

[HTTP 201 Created](http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html) returns the `Location` header of a new resource.  This pull request special-cases 2xx range responses where there's no body, but a location header is present.  This allows service interfaces to return the id of a new resource without leaking `retrofit.client.Response`.

ex.
```
/**
 * returns id of the new resource created.
 */
@POST("/") String create();
```