Time synchronization with server

- wrapper for /v2/public/time rest endpoint
- synchronize time before trying to authenticate a request in order
  to avoid invalid authentication because prevented replay attacks.

Resolves #1

lib/rest-client.js

@@ -121,4 +121,12 @@ module.exports = class RestClient {
   async getLatestInformation() {
     return await this.request.get('/v2/public/tickers');
   }
+
+  async getServerTime() {
+    return await this.request.get('/v2/public/time');
+  }
+
+  async getTimeOffset() {
+    return await this.request.getTimeOffset();
+  }
 }