Fixed an issue with GoogleAnalytics-iOS-SDK. For users that want to enable IDFA collecting, they can't because libAdIdAccess.a cannot be retrieved via the spec.
The fix involves creating a Core default podspec with all the files and configs we used to have. The IDFA subspec depends on the Core subspec and only adds the libAdIdAccess.a lib.
AdSupport framework should be weak, since GoogleAnalytics can run on iOS5 - I didn't manage to set a 6.0 requirement for IDFA subspec ([!] The platform of the target `Pods` (iOS 5.0) is not compatible with `GoogleAnalytics-iOS-SDK/IDFA (3.10)` which has a minimum requirement of iOS 6.0.)
Merge pull request #12825 from bpoplauschi/google_analytics_idfa_subspec

GoogleAnalytics-iOS-SDK subspecs+IDFA