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 allows us to receive and use URI data in response to restful services that use 201 semantics.

ex.
```
/**
 * returns id of the new resource created.
 */
@POST("/") URI create();
@GET("/") Thing get(URI id);
```