Experimental data change listeners support

• We've added a way to bind live event listeners to your Pipedrive account. Once a data you have access to changes on the Pipedrive account, your provided callback will fire in your code. Technically, the Client instance has a new .on(event, function) method. You can invoke it with a event pattern (similar to push notifications, e.g. "deal.added", and a callback function that will receive the following arguments: event, data) While this is experimental at this stage, we do not recommend running it in production just yet. For a real world example, see live-updates.js
• A few very minor changes. Very few, very minor.