Sequence looping now implemented with multiple note buttons

* Adding play button and multiple note buttons to view
* Play button toggles sequence playback on and off
* Note buttons toggle playing of note during sequence at appropriate time
* Bug causes view to crash and/or error message (Uncaught SyntaxError: audio resources unavailable for AudioContext construction) if more than one note is set to play and sequence loops
Looping crash/error fixed

* Fixed crash by only loading plugin once instead of every time a note is played
* Added initGrid that appends to the DOM a grid of 16x16 buttons
* Currently only the bottom row of buttons works since value of time sent to playNotes only corresponds to the array indices in that row
Merge pull request #4 from artfoundry/implement-looping

Implement looping