Added podspec for ReactiveCocoa v1.3.1

I used a copy of the podspec for v1.0.0 as a starting point.  I
increased the version and added more files to exclude from the iOS
deployment target.
Merge pull request #1472 from umjames/add_ReactiveCocoa_v1.3.1

Added podspec for ReactiveCocoa v1.3.1