iOS OSS: added CocoaPods setup to RNTester and fix up the podspecs

Summary:
This adds a way to test out CocoaPods build to RNTester, but as a separate project/workspace.
This also fixes the podspecs due to Fabric stuffs.

Note that this setup is meant to be run manually, not by CI (yet)

Reviewed By: mmmulani

Differential Revision: D7603823

fbshipit-source-id: 2219aa4c77f40ec07097a5c9ed4052529226618b