Big simplification to dragging directive.  Moved the offset calculation out of the directive and into application code.
I had to do this because to calculate the offsets reliably for SVG elements under firefox I had to use the SVG API instead of the jQuery
API I'd usually use to compute offsets of the HTML elements.  I didn't want to hard code the dragging directive to work with either HTML or
SVG (it should work with both) so now only the page coordinates are passed through to application code and it is resonsible for figuring out the
best way to compute the offsets if it needs them.  In this case the application code uses the SVG API to compute the offsets.
I also discovered that $element can be passed to a directive controller and is the root element for that directive, nice.