Add support for the specification of dependencies by configuration

Podfile parsing of the functionality for https://github.com/CocoaPods/CocoaPods/pull/1668 .

@irrationalfab has opinions on the syntax. I built what @alloy suggested. I think @alloy's syntax is easier for beginners (I'm thinking from the perspective of being the author of http://lookback.io/docs/install-using-cocoapods , where I want to make step #2 as short as possible). I also agree that it's weird to overload a hash whose primary usage is to specify where the code is, with linker settings. I think the usability argument is stronger, though. Both syntaxes could easily be supported.

I've written the code so it should be easy to switch to @irrationalfab's syntax, or to support both. Where do we go from here? (Also, what should I fix in my PR before it's mergeable? :) )