Add podspec.

Cocoapods (http://cocoapods.org/) is the dependency manager for Objective-C projects, very much like bundler for ruby. It uses a podspec file to define how the dependencies should be build and integrated into Xcode projects. I'm currently maintaining my own protobuf-c fork to support this (https://github.com/maxsz/protobuf-c), it would be nice to have it in the original fork.

I don't know if you're willing to add this, but I thought I just ask :)