Add a Cocoapod

It would be really cool to have a [CocoaPod](http://cocoapods.org) to refer to the source files. I'm building a tool on top of it [for RubyMotion](https://github.com/usepropeller/motion-screenshots), and having the dependency as a CocoaPod would be handy

Only thing you have to do is tag a commit as '0.0.1', which is a good idea for [releases](https://github.com/ksuther/KSScreenshotManager/releases) anyway

It seems that the practice for CocoaPods that you don't want included in release builds is to add them to a separate target, which I've added some small instructions for.

Totally at your discretion, appreciate any thought you'd give this!