added cocoapods support

replaces our .gitmodules with cocoapods and added Podspec for publishing.

utilized in the cookbook example: aerogear/aerogear-ios-cookbook#53

Note: not to be merged since the pod spec [points](https://github.com/cvasilak/aerogear-ios-httpstub/blob/podspec/AGURLSessionStubs.podspec#L9) to the private repo but once we are happy with the changes, will update to point to the official repo.
let's remove "but tests shown that it's pretty stable to use."