Removed dependency on TouchJSON, fixed warnings with stricter compiler settings

- The dependency on TouchJSON is no longer necessary on iOS 5+. So, I swapped it out for Foundation's NSJSONSerialization class.

- I noticed that when adding MoPub to my Xcode project, I was getting a bunch of compiler warnings. This is because I build my project with stricter warnings enabled, as compared to a vanilla Xcode configuration. Most of the warnings about MoPub were pretty harmless, but there was one error where the MPAdType enum was typed as NSUInteger but contained a negative number, as well as some other minor type conversion issues that must have slipped through. I fixed most of the warnings and silenced others with inline ignore directives to clang. Other users with strict compiler settings like mine may appreciate these changes.