API Change, Rests, and Sawtooth Waves

This release redefines the Hum language API such that octave numbers roll over on C natural per standard musical convention. It also introduces the ability to insert rests (silence) into the score, as well as a new voice: the sawtooth wave.

Behind the scenes, I made an effort to refactor the code to make certain functions more generic. In the future, expect support for defining custom scales alternative to the standard 12-note arrangement.

In the next API change, I will improve how command-line arguments work to make the workflow easier, as has been requested in the "Issues" section.