Add beatracker

[oubiwann]

After looking at photos of Småland this morning, I became inspired and an explicit design for this long-standing wishlist item finally solidified in the old brain pan:

Infrastructure tasks:

• beatracker: Add a gen_server to track state #20
• beatracker: Add a gen_server to track time/beats #21
• beatracker: Create an API for dynamically adding children to the supervision tree #22

Features:

• beatracker: Provide the ability to start and stop music playback #23
• beatracker: Provide the ability to start and pause playback of individual music phrases #24
• beatracker: Create an API for dynamically adding musical phrases #25
• beatracker: Provide a mechanism for listing all beatracker phrases #26
• beatracker: Provide a means of removing phrases from beatracker #27
• beatracker: Provide the ability for users to export beatracker sessions as MIDI files #28

Note that stopping playback on the beatracker and then resuming play will cause everything to be played over again, exactly as originally entered.

This design will be the long-sought union / convergence of live play (a la Extempore, SuperCollider, Overtone) and composition (choose your DAW or music notation software) ... essentially providing a very real REPL for musical composition and live-play/playback.