(Issue #4) Fixed: Plus/Minus keys change the volume

Python implementation of PacketBUTTON differs from the C++ implementation:
It checks for parameter "amount" beeing != None instead of >0.
If it is other than "None" (e.g. the default "0"), the flag "BT_USE_AMOUNT" is set, which conflicts at setting volume in XBMC.
Merge pull request #6 from herrminni/patch-1

(Issue #4) Fixed: Plus/Minus keys change the volume