In PostaggedToken, represent POS using a class

POS at present is represented as a symbol. This limits code reuse. I would like to add POS information to my own objects without using `PostaggedToken`. But by doing so I lose functions such as `isNoun`. If POS were represented as instances of a class I would get much greater flexibility. Another alternative would be to have the POS methods in a helper object, but that would not bring the type safety of an explicit POS type.