Improve positioning

I added a check to use getBoundingClientRect when available.

getBoundingClientRect is a native function which is available in almost all browsers since IE5/FF3 I think and is much faster then the jQuery functions (see here: http://jsperf.com/getboundingclientrect-vs-jquery).

The jQuery functions also don't work with svg elements, the getBoundingClientRect function does. This was my main reason for this change.

Feedback is welcome. :)