missing([timeout] [,success] [,message])
Waits until no elements are matched by the selector. Missing is equivalent to calling
//waits until #foo leaves before continuing to the next action. F("#foo").missing()
overrides FuncUnit.timeout. If provided, the wait will fail if not completed before this timeout.
a callback that is run after the selector exists, but before the next action
if provided, an assertion will be passed when this wait condition completes successfully
returns the funcUnit for chaining.