fixed crashes when midi is used anywhere else in the app

Dear sydlawrence,

I find your midi-launchpad package awesome, except for one thing: node.js mysteriously crashed more ofter than not when trying to use midi for other purposes as well in the app. (I am using OS X Mavericks)
Being able to supply my own instance of midi.input and midi.output solves the problem.
I ask you to accept this pull request.

Cheers,
Zsolt