Adding YearMonth support

@lucascs Sorry about too long delay. I don't like to force a fixed pattern like `MM/yyyy` because all other time converters are flexible. Take a look here: https://github.com/caelum/vraptor-time-converters/blob/master/vraptor-javatime/src/main/java/br/com/caelum/vraptor/converter/LocalDateConverter.java#L71

So, in my opinion, I think that the best choise is leave the code using localized pattern instead of fixed pattern

What you thing about that?