Xcode 6: fix compile errors caused by new signature of -[NSObject description]

On the OS X 10.10 SDK, `-[NSObject description]` is now defined as a
property. Updated any of our own `description` properties to be declared
with the same attributes, and manually synthesize ivars where necessary.

Adds unit tests to ensure that the synthesized ivars are actually used.
Merge pull request #542 from kiwi-bdd/osx-description-fix

Xcode 6: fix compile errors caused by new signature of -[NSObject description]