added cocoapods support

replaces our .gitmodules with cocoapods and added Podspec for publishing.

utilized in the cookbook example: https://github.com/aerogear/aerogear-ios-cookbook/pull/53

Note: note to be merged since the podspec [points](https://github.com/cvasilak/aerogear-ios-http/blob/podspec/AeroGearHttp.podspec#L8) to the private repo but once we are happy with the changes, will update  to point to the official repo.