fixes

- FIX: missing references
extenstion metods

- ADD: BeginInvokeSafe() extension methods for DispatcherObjects
StartNewTask

- CHG: changed result of StartNewTask() extension methods (with
StartNewTaskFunc<T, R> as logic) to Task<R>
code cleanups
TaskHelper

- ADD: helper class to extend functionallity of Task.WaitAll() and
Task.WaitAny() methods