updated podspec ahead of a new release

Merge pull request tonymillion#45 from sethfri/master

Use NS_ENUM (+ Doc Fixes)

Version 3.1 Stable

Tagged for CocoaPods - Reachability is iOS/MacOS compatible. Reachabi…

…lity v3.0.0 uses GCD and ARC, meaning it is compatible with XCode 4.2, iOS 4.0+ and MacOS 10.6+