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 podspec [points](https://github.com/cvasilak/aerogear-ios-oauth2/blob/podspec/AeroGearOAuth2.podspec#L8) to the private repo but once we are happy with the changes, will update to point to the official repo.