Create a subclass of OAuth.SignatureMethod, with the given getSignature function.
function makeSubclass(getSignatureFunction) {
        var superClass = OAuth.SignatureMethod;
        var subClass = function() {
            superClass.call(this);
        };
        subClass.prototype = new superClass();
        // Delete instance variables from prototype:
        // delete subclass.prototype... There aren't any.
        subClass.prototype.getSignature = getSignatureFunction;
        subClass.prototype.constructor = subClass;
        return subClass;
    }