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-jsonsz/blob/podspec/AeroGearJsonSZ.podspec#L8)  to the private repo but once we are happy with the changes, will update to point to the official repo.