use getBoundingClientRect in _getDOMObjectPosition

Updates _getDOMObjectPosition to use getBoundingClientRect() to compute an element's position. This is the technique used in jQuery, Zepto, etc. The main benefit is that the location will be computed correctly for elements that are position:fixed, or inside a position:fixed element. It should also be a lot faster.

It has good browser support (http://www.quirksmode.org/dom/w3c_cssom.html), but isn't currently supported in jsdom (I checked with the latest version, 0.5.1). This caused some of the tests to break, so I the old implementation in as a fallback.

Should fix issue #79 and a couple others.