Fix volume control

Commit 8012d6cb4bb65a858105ef878c5b98d91b24e1cd was broken in several
ways. Since auto-detection did not work correctly it was replaced by a
user-editable variable. See #439.
INSTALL: Add note about libav* configuration
Merge branch 'master' of https://github.com/PromyLOPh/pianobar