Update podspec for new arc requirement

__Description:__
The change 10fbf74 on 4/1/2014 to `Add preprocessor check to UIPopoverController+Spec.m to ensure ARC is enabled` broke cocoapods compiling ability since the podspec was not updated to reflect the per file ARC settings - this PR fixes that problem. __Without this (or similar) change, pods users compilation will break badly if they try to use Core/SpecHelper/Stubs.__ I might be nice  to add a story to the backlog to create an integration test to ensure that changes like this to source do not break things for pods users.

Also, I've added Foundation+PivotalSpecHelper.h to the exported sources for Foundation/SpecHelper so pods users can simply import that in the pch (or wherever) like we all love to do instead of importing the individual spec helpers in foundation.

__Changes:__
- Fix podspec to support arc requirement in specific file
(UIPopOverController)
- Add Foundation+PivotalSpecHelper.h to exported source so it
is easier to import the Foundation spec helpers in general.