Track object encapsulates loading and playing of one audio file
renamed audio_context to track.js for better clarity
added offset and looping capabilities. And multiple tracks
Fix merge conflicts
slide and shorten on click
Merge pull request #1 from generalzhou/playing_tracks

created track object
Fix merge confict in layout
Merge pull request #3 from generalzhou/shorten_slider

slide and shorten on click
merged with master