Added selection (d3.js brush) to SVG Context.
Builder propagates selection events to contexts.
Added basic brush API (needed by SVG context) to tracks
Removed old CSS.
Added brush extent to stylesheet.
Created prototype object for tracks.
Moved guid setter and height getter/setter functions to track prototype.
Cleanup
Added buttons for enabling zoom/selection to example
Implemented toggleZoomMode and toggleSelectionMode
Implemented finding unique ids from selected coordinate range.
Updated example code to log selected ids.
Moved brush-related logic to prototype
Updated example code