Split iMpulse.js into its separate classes and concerns

Created src/main/javascript
Moved iMpulse into own class
Moved Stream to iMpulse.Stream
Changed Stream.streams to iMpulse.sequences
Moved Signal to iMpulse.Signal
Moved PadEvent to iMpulse.PadEvent
Restored demo as web test
Updated README to tell contributors how to build