slightly amend plugin requires (#419)

BPD does not need requests (it's vendored), but it does need pygst.

setup.py

@@ -80,19 +80,19 @@ setup(name='beets',
       + (['colorama'] if (sys.platform == 'win32') else [])
       + (['ordereddict'] if sys.version_info < (2, 7, 0) else []),
 
-      # plugin requires
-      extras_require = {
+      # Plugin (optional) dependencies:
+      extras_require={
           'beatport': ['requests'],
-          'bpd': ['bluelet'],
           'chroma': ['pyacoustid'],
           'discogs': ['discogs-client'],
           'echonest_tempo': ['pyechonest'],
           'lastgenre': ['pylast'],
-          'web': ['flask']
+          'web': ['flask'],
       },
-      # non python plugin requires
+      # Non-Python/non-PyPI plugin dependencies:
       # replaygain: mp3gain || aacgain
       # convert: ffmpeg
+      # bpd: pygst
 
       classifiers=[
           'Topic :: Multimedia :: Sound/Audio',