[soundcloud] Adding likes support to SoundcloudUserIE

This commit adds support for downloading a user's liked tracks.

Basically, i have modified `SoundcloudUserIE` to also accept likes page url. If the url points to a likes sub-resource then we simply switch the API call `base_url` to `/favorites` instead of `/tracks`.

Additionally, i have modified the pagination loop break. Sometimes (especially towards the older pages), the soundcloud API seems to be sending fewer than 50 entries even when there are more pages left. The loop will now run until a zero entry response is received.

I have also added a test in `test_playlists.py`