Render element regardless of its position on the page

Added options.renderRootAtOrigin flag, when set to true the element being rendered will be rendered at 0,0 instead of at its actual page position. However, as it is now the resulting image will be the full page's dimensions instead of the target element's.

Haven't yet figured out exactly where the final image size is determined, so the resulting image needs to be cropped. Not sure what will happen if the element to be rendered is taller than whatever size is currently chosen for the image.