KWValue support for stdbool.

KWValue could only wrap Objective-C BOOL type, but lacked support for
stdbool. As a result, `-[KWValue boolValue]` would raise an exception
when invoked with a wrapped `bool` value.

Add support for `bool` wrapping.

- Rename `KWObjCTypeIsBool` to `KWObjCTypeIsBoolean`, and have it return
  `YES` for both `BOOL` and `bool`.
- Add `NSNumber` conversion method for `bool`.
- Add tests.

Fixes issue #506.