feat(util): add object with utilities methods

These methods are only available in the wams.js library, are not
available for user.
feat(util.offset): method returns offset of element on page

Method gets element and returns element's left and top offsets.
It is calculating by adding offsets of element and all its parents.
feat(util.isDraggable): returns if element can be dragged

SOLUTION IS TEMPORARY

This method returns if element can be dragged on the page. This method
is to prevent bug when multitouch elements lose focus and user start
dragging <html> or <body>.
Method checks presence of class 'ball', for bigger portability we should
check instead that this object is among MTObjects array.
feat(wams.on): send offset of element, not touch point

Previously with element position touchpoint was sent, and to find offset
of element, it was necessary to subtract touchX and touchY coordinates.
Currently, actual offset of the element is sent over network.
fix(wams.on): check if element is draggable

This check here to prevent bug, when focus was changing from multitouch
element to <html> or <body>.