Instance variables (numberFormatter, dateFormatter) on singleton not thr...

...ead safe.

Note NSDateFormatter, and NSNumberFormatter are not thread safe classes: 
https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/Multithreading/ThreadSafetySummary/ThreadSafetySummary.html.

Accessing the singleton from multiple threads seems to be causing an issue when both threads are using the self.dateFormatter property. Didn't see the same issue with self.numberFormatter but assuming it exists as well.