Better onBeforeStart for drag

[DanielSteigerwald]

Please, move this line:

this.fireEvent('onBeforeStart', this.element);

in drag.js after (by one line)

this.mouse.start = event.page;

Would be handy to have mouse position in onBeforeStart handler

start: function(event){
    if (this.options.preventDefault) event.preventDefault();		
    this.mouse.start = event.page;
    this.fireEvent('onBeforeStart', this.element);