Added observer for opening an email. Closes #2

Added  observer **`open_email`** that is triggered when a user loads an email from any of the inbox views.

Returns: 

- email_id of the requested email
- captured request parameters that were posted to the server
- returned post response 

```js
gmail.observe.on("open_email", function(id, url, body) {
   console.log("id:", id, "url:", url, 'body', body);
})
```