Fixed the implementation of wait

The 2.0 release of Zombie changed the behavior of ``browser.wait(function)`` to consider it as the callback rather than the completion function, thus not returning a closure.
The completion function is not passed in the options, together with the duration instead of changing the global setting. The callback is also passed directly rather than using the promise-based API to avoid [a bug](https://github.com/assaf/zombie/issues/755) in Zombie 2.0.0 to at least 2.0.3 when using wait() without opening a page.