Ensures priority returned is a number or increments it's internal priority counter
and returns it.
@param priority {Number}
@method sjl.stdlib.PriorityList#normalizePriority
@returns {Number}
function (priority) {
            var retVal;
            if (sjl.classOfIs(priority, Number)) {
                retVal = priority;
            }
            else {
                this._internalPriorities += 1;
                retVal = +this._internalPriorities;
            }
            return retVal;
        }