single and double click event handlers - singleDblClick builds on the dblClick feature

[nphias]

addresses: Feature request 286

ability to have event handlers for both single and double click - singleDblClick builds on dblClick by adding two more event handlers .. beforeSelect and beforeDeselect
example usage:

$('#my-select').multiSelect({
            singleDblClick: true, 
            beforeSelect: function(values){}, 
            beforeDeselect: function(values){},
            afterSelect: function(values){},
            afterDeselect: function(values){}
       })