Provide CocoaPods subspecs for individual locales

[1ec5]

The CocoaPods podspec should define subspecs for individual locales, so that a size-conscious developer can pick-and-choose the locales that their application supports.

To avoid hard-coding each supported locale in this repository – one of the goals of #14 – we could provide a subspec for English, one for the Chinese locales, one for Western European locales, and one for all remaining languages. By default, all the subspecs would be included.

In each subspec, json2plist.sh would accept locale codes as arguments and convert only the specified locales if provided.

/cc @frederoni @bsudekum @freenerd