Features/delete user

This PR allows client code to delete a user by sending a valid DELETE HTTP request. Listed below are HTTP request and response body.

Valid  DELETE request
----------------------------------
~~~shell
curl -X DELETE http://localhost:9393/api/v1/users/the_dude
~~~

~~~json
{
  "id": 16,
  "name": "the_dude",
  "email": "hisdudeness@example.com",
  "bio": "Memeber of the SLF",
  "created_at": "2013-10-04T00:39:20Z",
  "updated_at": "2013-10-04T14:49:20Z"
}
~~~

DELETE request for a non-existent user
--------------------------------------------------------
~~~shell
curl -X DELETE http://localhost:9393/api/v1/users/donnie
~~~

~~~json
{
  "error": "User not found"
}
~~~