beatracker: Create an API for dynamically adding musical phrases

[oubiwann]

Notes:

• phrases will be stored in the state gen_server for beatracker
• the timer gen_server for beatracker needs to be independent of musical phrases

Depends on:

• beatracker: Create an API for dynamically adding children to the supervision tree #22

Blocks:

• beatracker: Provide a mechanism for listing all beatracker phrases #26

Part of:

• Add beatracker #19