Add support for delegating usage

For #59.

Dynamic is very common. I made a modification in my fork:

https://github.com/mytharcher/html5sortable/blob/master/jquery.sortable.js

@webstrap your fork can't work because the variable `items` in code (near line 26) can't be initialized with dynamic elements. So I move the assignment of `items` to `dragstart` event handle function. But cause a disadvantage which the `draggable="true"` attribute can't be initialized automatically.

Anyone like this feature could merge this branch. Just **make sure your dynamic elements have already contained the `draggable="true"` attribute** when you inserting them to DOM.