Added podspec file for cocoapods

Hello! 
I've found that podspec for 'CedarAsync' is already exists on [CocoaPods specs](https://github.com/CocoaPods/Specs/blob/master/CedarAsync/0.0.1/CedarAsync.podspec). But it doesn't work with new version of [CocoaPods](http://cocoapods.org/) - 0.22.x. It's because of C++ compiler options.
So, I've made new podspec and it works fine! And it would be great to add it repo =)

So, if you merge this pull request then you can contribute CedarAsync podspec to [CocoaPods specs repo](https://github.com/CocoaPods/Specs). Or I can do it for you. Just ask me =)

Also, I don't know your email address, and you can update podspec file with it.