Add podspec for CocoaPods

CocoaPods is the primary dependency manager for iOS and Mac OS.

Including this file in the repository is not required to use libbson in CocoaPods, but it makes it simpler for developers if they fork libbson, and makes it more convenient to publish a new spec when the library releases.
Can s.description be a link to https://github.com/mongodb/libbson#libbson?