Allow creating custom NSFormatters

This pull request has two parts (monomac and maccore).

In combination with data binding this allows to get the standard "Please provide a valid value." dialogs.

Please note:

* I added a sample in monomac (CustomNSFormatter)
* I had to implement getObjectValue:forString:errorDescription: in a subclass of NSFormatter, otherwise it will raise an exception, complaining that is not implemented in a subclass. (Probably this could also be solved with some ObjCRuntime magic.)
* I called that class NSFormatterDelegate, which is not completely correct. It is used as a delegate class but not for NSFormatter. You know a better name?
* I could mark the two methods that must be implemented when subclassing NSFormatter abstract. Which is nice because developers can't get the exceptions anymore.
* The design is not completely standard, but the problem wasn't standard either.