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
in Zombie 2.0.0 to at least 2.0.3 when using wait() without opening a
page.
Merge pull request #112 from stof/fix_waiting

Fixed the implementation of wait