Switch to storing strings and defaultlanguage as @properties

Switched to using @properties to avoid issues when running when using CocoaPods. Without this change, when the code is run the `_strings` ivar seems to get nulled out, leading to a bad access error as the app is used. I'm not sure exactly what causes this, but perhaps including it as a Pod (and thus as a separate build target) means that ARC doesn't know if it should hold onto the reference and erroneously removes it. Switching to @properties means that the MCLocalization singleton should hold onto the `strings` reference while it is in memory.