Add OS X support to Podspec.

This was a pretty painful process. Despite the project having separate OS X and iOS source folders, there are files in the source root that do not belong there. In the pod spec is a list of files that do not compile on OS X and should probably be moved to the iOS folder. I will let you guys decide how you want to handle this. Additionally, GPUImage did not compile with default Xcode build settings. Namely `-Wreturn-type`. `GPUImageView` has a function that does not evaluate all options in its switch statement before retiring and also does not provide a default case, which throws an error. I was unable to silence this error from the `compiler_flags` option in the spec, so a code change was necessary. We should also advance the tag and spec version to the next version and push to CocoaPods.