Support dragging of SVG elements

Fixes #7.

* Fallback to emulated drag mode if an SVG element is dragged.
* IE9 doesn't include some methods for SVG elements. In these cases, I've had to implement workarounds.
* Adding emulated and non-emulated dropzone behavior. DropzoneGroup doesn't know if an SVG element is being dragged.
* Tests added to `test/svg` folder.
Add test for draggable within an SVG
Move feature detection to html5_dnd.dart and utils to utils.dart

Also add the contains() helper method to utils.dart and use it whenever
svg elements could be involved.
I think it's ok to always install emulated dropzone
Handle cloning of SvgElement to create a drag image