[webhook,polling] Improve starting, stopping of webhook, polling

Feature:

  The different mechanisms of fetching updates, i.e. polling
  and webhook, have had their implementations improved:

  * the TelegramBot instance needs to create the polling and
    webhook instances once, and when necessary
  * returning promises from TelegramBot#openWebHook() and
    TelegramBot#startPolling() allows more precise control

  Also,

  * TelegramBot#initPolling() is being deprecated in favor of
    TelegramBot#startPolling() to ensure consistency (as the
    opposite action of TelegramBot#stopPolling())

test/telegram.js

@@ -178,10 +178,11 @@ describe('TelegramBot', function telegramSuite() {
     });
   });
 
-  describe('#initPolling', function initPollingSuite() {
+  describe('#startPolling', function initPollingSuite() {
     it('initiates polling', function test() {
-      testbot.initPolling();
-      return utils.isPollingMockServer(pollingPort);
+      return testbot.startPolling().then(() => {
+        return utils.isPollingMockServer(pollingPort);
+      });
     });
   });
 
@@ -213,8 +214,9 @@ describe('TelegramBot', function telegramSuite() {
 
   describe('#openWebHook', function openWebHookSuite() {
     it('opens webhook', function test() {
-      testbot.openWebHook();
-      return utils.hasOpenWebHook(webHookPort);
+      return testbot.openWebHook().then(() => {
+        return utils.hasOpenWebHook(webHookPort);
+      });
     });
   });