BPM: disable extension by default (bgo#722551)
BPM: disable extension by default (bgo#722551)

BPM detection is kind of a niche feature and, as it has already
caused so many crashes (due to the many bug reports we got[1]),
it's safer to not enable it by default.

[1] See all duplicates of
https://bugzilla.gnome.org/show_bug.cgi?id=722551

GNOME/banshee,Carbenium/banshee,GNOME/banshee,arfbtwn/banshee,arfbtwn/banshee,GNOME/banshee,arfbtwn/banshee,arfbtwn/banshee,arfbtwn/banshee,GNOME/banshee,Carbenium/banshee,arfbtwn/banshee,Carbenium/banshee,GNOME/banshee,GNOME/banshee,Carbenium/banshee,GNOME/banshee,arfbtwn/banshee,Carbenium/banshee,arfbtwn/banshee,Carbenium/banshee,GNOME/banshee