Generic delegate callbacks

[ewanmellor]

Add a new feature to call parser:willMatch: or parser:didMatch: on the delegate whenever a rule is matched.

This adds a new PKParserDelegate protocol, so that the selectors are declared (though PKParser.init does not require this protocol to be implemented by its parameter, because this whole protocol is optional).

Move the parser:didFailToMatch: declaration to PKParserDelegate, now that we have such a thing.