GoogleAnalytics-iOS-SDK subspecs+IDFA

### Please do not merge yet
@alloy @x2on, please look at this first, as you have pushed all the previous specs. If this sounds like a legit fix, we can apply it to previous specs as well.

#### The change
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` along with a dependency to the `AdSupport` framework.
Agree. The `IDFA` subspec should require at least iOS6. I will update the PR