Allow for an optional drag handle to be specified

[manavo]

When defining the gridster, you can now define a draggable.handle selector:

For example:

    $("#widgets").gridster({
        widget_margins: [10, 10],
        widget_base_dimensions: [140, 140],
        draggable: {
            handle: '.title'
        }
    });

If handle is omitted (or left blank), it works as it did previously. If a value is specified, then dragging only starts if the element which was clicked matches the selector specified.

[vieron]

Thanks for the PR @manavo!

I made the code a bit shorter and now draggable.handle supports both selectors as strings and DOM objects.

You can see it merged with master here 552df22