Upgraded RtMidi library to version 2.0.1

Upgraded from official package found at:
http://www.music.mcgill.ca/~gary/rtmidi/

This new version includes support for JACK Audio and Windows Kernel Streaming. Also the ALSA API replaced a sleep-loop with polling, making this backend much easier on CPU. I use node-midi mostly on a Raspberry Pi and performance is noticeable better with this version.