Added MTJSONUtils.

This is from https://github.com/mysterioustrousers/MTJSONUtils,
eb5dc05f5e, Copyright (c) 2012, Mysterious Trousers, MIT license.
Remove the NILL, NUL, NULS macros.

I'm not a fan -- I think the names are confusing.
Fix some type-safety errors.
Add NSObject+MTJSONUtils to the project.
Add stringWithJSONObjectMadeSafe x 2.

These are the same as stringWithJSONObject, except they use
objectWithJSONSafeObjects from MTJSONUtils to replace any
NSDate objects with their ISO 8601 equivalent (and other safety checks).
Fix the delayed synchronize call if the setting is updated from a
background thread.

We need to dispatch the performSelector call onto the main thread,
because the background thread might not have a run loop, and
performSelector needs one.