Revised code to use CocoaPods rather than Git submodules

I revised your AFNetworking-RACExtensions project to use CocoaPods rather than Git submodules. I wanted to work on adding support for AFNetworking v2.0 _and_ be able to test the changes in an existing project that uses CocoaPods, so this seemed like a good first step.

One consequence of the change to CocoaPods is that you'll need to open `Reactive AFNetworking Example.xcworkspace` rather than `Reactive AFNetworking Example.xcodeproj`. Failure to use the workspace will result in strange build problems.

While I was updating the project, I also made some edits to `README.md` and included a copy of the `AFNetworking-RACExtensions.podspec` file so that people can point directly to your GitHub repo. I bumped the version embedded in the podspec to 0.1.3 and created a corresponding tag in my repo.

The only thing I couldn't figure out is how to get the `RAFNetworking Tests` to run. There were numerous targets in the original project, but I couldn't seem to find one that ran the tests. If you can point me in the right direction I'll be happy to take a stab at it. For the moment, the example project only builds the iOS sample code.


-- Scott