Don't depend on clientWidth and clientHeight

clientWidth and clientHeight don't get intialized properly on top-level <svg>s
in Firefox (FF bug #874811). Instead, the top-level Component now reads the
width and height attributes from the svg. main.js has been added to properly
set these properties. In addition, Plottable.Utils now has a function
getSVGPixelWidth() that performs the same calculation. That call can be
added to Component.computeLayout() if necessary.