Implement endpoint for GETting a user

This PR allows client code to GET a user by name. Listed below are HTTP request and response body.

~~~shell
http://localhost:9393/api/v1/users/the_dude
~~~

~~~json
{
  "id": 16,
  "name": "the_dude",
  "email": "dudeabides@example.com",
  "bio": "El duderino",
  "created_at": "2013-10-04T00:39:20Z",
  "updated_at": "2013-10-04T00:39:20Z"
}
~~~