Fix tests where the compiler fails to infer types.
Override beEmpty() to deal with ambiguity.

Alternatively, we could remove the NMBCollection variant and rely on
countElements/CollectionType.
This does not infer correctly in 6.1b3.
Correct the reference to inferencing in the readme.
Okay Xcode, whatever you say.
Merge pull request #42 from github/xcode-6.1b3-type-inferencing-correction

Fix tests where the compiler fails to infer types.