[arch-general] mpd had removed relative volume functionality.

[b4283]

According to developer Cirrus, the relative volume adjusting method has 
been deprecated for 5 years.
which is finally abandoned in the git version. (does anybody know why 
mpd in the repo hasn't been updated for sometime while stable 0.15.1 is 
around?)

So anyone who uses mpc to bind a hotkey to shortcut such as "mpc volume 
-5" or "mpc volume +10" is likely to get a error from mpd.

I've written a script to realize such functionality, please follow: 
http://dpaste.com/70907/
save it as script, chmod +x to it and put the relative values as parameter.

E.g. "vol.sh +5" or "vol.sh -10"

--
regards,
b4283