iOS 6.1 SDK fix

This commit allows the category to be built against the iOS 6.1 SDK. Before compiling would fail due to the `topLayoutGuide` and `bottomLayoutGuide` properties not existing in the 6.1 SDK.

I've also added the `__unused` attribute to a few variables that are only used in macro checks to prevent release mode compile time warnings.

Podspec version has been increased to 0.0.8