NSDateFormatter string literals in format strings needs to be escaped

The NSDateFormatter extension methods use format strings that contain literal text that is not properly escaped. Literal text in the format strings should be enclosed in apostrophes, per the [Data Formatting Guide documentation on NSDateFormatter](https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/DataFormatting/Articles/dfDateFormatting10_4.html#//apple_ref/doc/uid/TP40002369-SW13), which in turn relies on [Unicode Technical Standard #35](http://www.unicode.org/reports/tr35/tr35-25.html#Date_Format_Patterns)

I added single-quotes/apostrophes to all literal text.