Rollbar.prototype.wait() for browser envrironment

[mstssk]

Rollbar.prototype.wait() is missing in browser implementation.

It would seem that the same implementation should be done on the server side.

rollbar.js/src/server/rollbar.js

Lines 295 to 305 in 31db200

	Rollbar.prototype.wait = function (callback) {
	this.client.wait(callback);
	};
	Rollbar.wait = function (callback) {
	if (_instance) {
	return _instance.wait(callback);
	} else {
	var maybeCallback = _getFirstFunction(arguments);
	handleUninitialized(maybeCallback);
	}
	};

Unless there is some special reason, I will create a PR.