Support calls from client / browsers

Most modern browser will issue a pre-flight request with HTTP OPTION method before the actual asynchronous POST.    This adds a CORS supporting option endpoint.

packages/rocketchat-integrations/server/api/api.coffee

 Api = new Restivus
-	enableCors: false
+	enableCors: true
 	apiPath: 'hooks/'
 	auth:
 		user: ->