Fixed [[object should] beNil] and tests with similar syntax

This PR includes changes from https://github.com/allending/Kiwi/pull/304. Please merge/close that one first.

This is not a particularly pretty change, but it seems to be working, and will allow us to get rid of the `KWExistVerifier` once we remove `shouldBeNil` and `shouldNotBeNil`.

Temporarily, I added specs for this to the 'integration test file'. This should be followed by an effort to write tests for Kiwi, in Kiwi (in addition to a slim SenTestingKit/XCTest suite that assures basic functionality).

Starting slowly though, I don't want this to happen:

![Gentle touch](http://25.media.tumblr.com/897c92ebbe2b42e3b40a0688fe4b16ae/tumblr_mmswijgIej1s917bwo1_400.gif)