Change default to use ARC and specify on per file basis if it's disabled.

This change makes the default settings for pod projects to have ARC enabled via `CLANG_ENABLE_OBJC_ARC`. Pods that don't require ARC uses the `-fno-objc-arc` flag for each file to disable ARC. 

This should from what I can see and the specs pass with the updates in the [sister PR](https://github.com/CocoaPods/cocoapods-integration-specs/pull/14), however I am sure I could have missed something. The semantics regarding `requires_arc` are slightly strange now imo. `requires_arc = false` doesn't imply that ARC is forbidden in the same sense that `requires_arc = true` implies that ARC is strictly required.