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.