Added instance for ArgVal Word

Please consider applying the included patch, which contains an instance of ArgVal for the Word type.  This instance is very useful because it allows one to enforce that the input be non-negative.  To implement it, I added a parser pNonNegativeInteger that is just like pInteger but without the possibility of a sign;  I then used this parser to create an instance of ArgVal for Word that looks essentially the same as the instance for Int.