Added podspec file for cocoapods

[belkevich]

Hello! I've found that podspec for 'CedarAsync' is already exists on CocoaPods specs. But it doesn't work with new version of CocoaPods - 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. 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.