fix(connection): allow more connection callbacks

Node's default limit for max listeners on an event emitter is at a low
10. We are using an event emitter internally in the Connection class to
handle queueing callbacks for the token retrieval event. The default of
10 is now lifted to unlimited, so the developer doesn't have to worry
about overriding the default themselves.

Fixes googleapis#223

lib/common/connection.js

@@ -94,6 +94,7 @@ module.exports.Token = Token;
  */
 function Connection(opts) {
   events.EventEmitter.call(this);
+  this.setMaxListeners(0);
 
   opts = opts || {};