Change argument order for on/off/delegate

  - on(element, event, handler) is easier to remember. It is not
    necessary to put the element last in this case as it is unlikely
    that partially applying on(event, handler) is going to be useful.
    The reason for this is that we expect handlers to mostly be a tiny
    bit of inline glue to some other code.
  - delegate takes the delegator first so it can be partially applied as
    a special purpose event delegator