Need to be able to remove instrument
Revert "Need to be able to remove instrument"

This reverts commit 0effef82e7ed8b28a91f765fdb20db5a1ea7941e.
Default onFinishedCallback

By default, the onFinishedCallback is undefined, which can give errors when play() is finished. Line 68 now checks to make sure cb is a function.
Ability to start at any given beat
Merge branch 'removeinst' of https://github.com/eugene-bulkin/8bit.js into removeinst