Instantiate a SignatureMethod for the given method name.
function newMethod(name, accessor) {
        var impl = OAuth.SignatureMethod.REGISTERED[name];
        if (impl != null) {
            var method = new impl();
            method.initialize(name, accessor);
            return method;
        }
        var err = new Error("signature_method_rejected");
        var acceptable = "";
        for (var r in OAuth.SignatureMethod.REGISTERED) {
            if (acceptable != "") acceptable += '&';
            acceptable += OAuth.percentEncode(r);
        }
        err.oauth_acceptable_signature_methods = acceptable;
        throw err;
    }