Track the amount of time the mouse has been held down for.
A bunch of garbage code for fading synths in and out as mouse depressed
Fix accidental space
A lot of refactoring, kill nodes that have faded out.

Refactors:
  - Make input class pure, move mouse-down-duration
  and mouse-down? into a single atom.
  - Move updating of active and releasing nodes into
  methods to improve readability of update method.
Properly release nodes that are finished playing.
Merge pull request #5 from akbiggs/mouse-down-duration

Fade synths in and out as mouse is held down/released