Add a browser.js file that just exposes MqttClient and MqttConnection.

I'm working on building a MQTT-over-WebSocket (and also engine.io) solution, and I needed a way to use this library in the browser, too.

This pull-request exposes only MqttClient and MqttConnection. I'll build two more packages that enable MQTT-over-WebSocket and MQTT-over-engine.io. 
This library is already big and I prefer keeping this 'web' functionality out of it.

@adamvr do you agree?