Fix encoding issues for track names, artist names, paths and search queries when they contain non ascii characters

I was trying to download a couple of playlists:
spotify:user:miguegs:playlist:0drqDMsC280akTPmFJhljj
spotify:user:raulcumplido:playlist:62SibcIG7nED4uLiRvcEcn

I´ve found that if the name of the artist or the track contains non-ascii characters it fails:
```
Traceback (most recent call last):
  File "/Users/raulcd/Envs/spotify-onthego/bin/spotify-onthego", line 9, in <module>
    load_entry_point('spotify-onthego==0.0.1', 'console_scripts', 'spotify-onthego')()
  File "/Users/raulcd/Projects/spotify-onthego/onthego/scripts/cli.py", line 24, in download_playlist
    skip_existing=(not args.no_skip), convert_to_mp3=(not args.no_convert))
  File "/Users/raulcd/Projects/spotify-onthego/onthego/download.py", line 17, in audio
    audio_file_path = onthego.youtube.download_to_tmp(track_name, artist)
  File "/Users/raulcd/Projects/spotify-onthego/onthego/youtube.py", line 17, in download_to_tmp
    print("    Downloading %s to %s" % (swf_url, tmp_path))
UnicodeEncodeError: 'ascii' codec can't encode character u'\xf1' in position 183: ordinal not in range(128)
```

This fixes the possible errors.