Added commands to login, show liked mixes,(un)like mix,(un)favorite track.

Changes in api.py:
Add obtain_user_token and post to log in to 8tracks.
Add get_mix_liked was added to show liked mixes by current user.
Add like_mix, unlike_mix, fav_track and unfav_track.

Changes in client.py:
Add login commands.
Added autologin function using config info in preloop (not sure if it's the right place to do so).
Added liked commands.
Added like_mix, unlike_mix, fav_track and unfav_track and aliases.
Added get_logged to check if user is logged in.

Changes in README.rst:
Added descriptions for new commands.