Drums Update

[DhrBaksteen]

This update makes a few changes to how drums are handled in percusive mode and adds a new example that makes use of the Piano Board.

• New definitions for the drum sounds. Old definition is renamed to DRUM_BITS.
• Added a variant on the setDrums function that allows you to update the bits of the drum register using a byte value.
• Added the playDrums function that allows playing a single drum instrument at a particular frequency.
• Added the PianoBoardDrummer example that turns the Cheerful Electronic Piano Board into a drum machine.