Apply most best practices of PEP 8

This refactoring includes:
- Two empty lines before each global function
- Using '{0} {1}'.format(str1, str2) instead of str1 + ' ' + str2
  Sometimes this will make lines longer, sometimes shorter.
- Starting all comments with # + space + comment
- Make lines not longer than 80 characters in most cases
- Renaming some variables to make more sense
- Add some missing code like returns and Exceptions

Not included, but follows:
- Make some comments docstrings
- Rename all 'file' variables, only for Python 2
- Remove some way too verbose comments ;)

test/test_single.py

@@ -4,22 +4,26 @@ import spotdl
 
 raw_song = 'http://open.spotify.com/track/0JlS7BXXD07hRmevDnbPDU'
 
+
 def test_spotify_title():
     expect_title = 'David André Østby - Intro'
     title = spotdl.generate_songname(raw_song)
     assert title == expect_title
 
+
 def test_youtube_url():
     expect_url = 'youtube.com/watch?v=rg1wfcty0BA'
     url = spotdl.generate_youtube_url(raw_song)
     assert url == expect_url
 
+
 def test_youtube_title():
     expect_title = 'Intro - David André Østby'
     content = spotdl.go_pafy(raw_song)
     title = spotdl.get_youtube_title(content)
     assert title == expect_title
 
+
 def test_check_exists():
     expect_check = False
     content = spotdl.go_pafy(raw_song)
@@ -28,12 +32,14 @@ def test_check_exists():
     check = spotdl.check_exists(music_file, raw_song)
     assert check == expect_check
 
+
 def test_download():
     expect_download = True
     content = spotdl.go_pafy(raw_song)
     download = spotdl.download_song(content)
     assert download == expect_download
 
+
 def test_convert():
     # exit code None = success
     expect_convert = None
@@ -45,6 +51,7 @@ def test_convert():
     convert = spotdl.convert.song(input_song, output_song)
     assert convert == expect_convert
 
+
 def test_metadata():
     expect_metadata = True
     content = spotdl.go_pafy(raw_song)
@@ -60,6 +67,7 @@ def test_metadata():
 
     assert metadata_output == (metadata_input == expect_metadata)
 
+
 def check_exists2():
     expect_check = True
     content = spotdl.go_pafy(raw_song)

test/test_username.py

@@ -4,18 +4,21 @@ import spotdl
 
 username = 'alex'
 
+
 def test_user():
     expect_playlists = 7
     playlists = spotdl.spotify.user_playlists(username)
     playlists = len(playlists['items'])
     assert playlists == expect_playlists
 
+
 def test_playlist():
     expect_tracks = 14
     playlist = spotdl.spotify.user_playlists(username)['items'][0]
     tracks = playlist['tracks']['total']
     assert tracks == expect_tracks
 
+
 def test_tracks():
     playlist = spotdl.spotify.user_playlists(username)['items'][0]
     expect_lines = playlist['tracks']['total']