Stop event propagation for {brush,drag,zoom}.

This allows behaviours and d3.svg.brush to be used together easily.

Also, only bind relevant event listeners on brush touchstart; a
mousemove event was being triggered on mobile Safari causing the brush
to jump briefly on touchstart, e.g. when viewing:

  http://bl.ocks.org/mbostock/raw/1667367/

Lastly, suppress user-select for brush.

Fixes #1321.