add voice region getting (#1108)

* add voice region getting

* clean up this mess

* add docstrings

* Update VoiceRegion.js

* Update RESTMethods.js

src/client/Client.js

@@ -298,6 +298,14 @@ class Client extends EventEmitter {
     return this.rest.methods.getWebhook(id, token);
   }
 
+  /**
+   * Fetch available voice regions
+   * @returns {Collection<string, VoiceRegion>}
+   */
+  fetchVoiceRegions() {
+    return this.rest.methods.fetchVoiceRegions();
+  }
+
   /**
    * Sweeps all channels' messages and removes the ones older than the max message lifetime.
    * If the message has been edited, the time of the edit is used rather than the time of the original message.