New Transformers for NSString to NSDate and NSDate to NSString

I added these new transforms to provide and __easy__  and __straightforward__ way to go between `NSDate` object and `NSString` representation of dates.

Features include:
* setting standard date formatter. _Right now it is hard coded in the `NSFormatter` category_
* cache/reuse of all custom formatters
* thread safe

_I also add `.gitignore` file_