Issue #7563 - Allow mocha timeout to cancel flow.

This pull request is an attempt to address the issues described in https://code.google.com/p/selenium/issues/detail?id=7563

The only problem I see with this is that you would need to add a timeout to each `it` step (and have no way to define a default). Mocha provides a built in way to define the default timeout for any step (`it`, `beforeEach`, etc.) so I would hate to invalidate that (and not provide an alternate way to specify it?) What do you think, would adding documentation about (set `mocha --timeout 0` flag to set default timeout to no timeout or something?)