Small optimization for parseUnsignedNumber

This implements a small optimization for the `parseUnsignedNumber` method in the parser. This method is called multiple times for parsing almost all packets, making a gain here seem like it would be pretty nice. Checking out [a simple test on jsperf](http://jsperf.com/parse-uint) it seems like it would make this method about 3x faster than what it is now. I cannot say if this actually has any real-world performance impacts yet, though.