Add Typescript typings

Author: Josh Woodcock

index.d.ts

@@ -0,0 +1,4 @@
+export declare class MessageValidator {
+    encoding: string;
+    validate(hash: any, cb: (err: string, message: string) => any);
+}

index.js

@@ -213,4 +213,6 @@ MessageValidator.prototype.validate = function (hash, cb) {
     validateSignature(hash, cb, this.encoding);
 };
 
+MessageValidator.MessageValidator = MessageValidator;
+
 module.exports = MessageValidator;