Improve unit tests

Work in progress do not merge.

Just if you want to see what I modify in your library to improve tests.
- I add SimpleCov to see more with code coverage.
- I remove username in user class, this data is never return by the API, l_name is always empty, only f_name is returned to identify the user name.
No, rubocop say: 

```bash
spec/grooveshark/playlist_spec.rb:89:39: C: Place the . on the next line, together with the method name.
    allow(client).to receive(:request).
```