Complementing support to /v2/events

[castanhob]

First of all, thanks for this amazing lib and work!

This pull request adds a required header for the /v2/events endpoint, which needs the Accept: text/event-stream to connect (following the Marathon API Documentation).

Not only that, but since request-promise currently has problems resolving promises with streams (check request/request-promise#90)
and even their documentation states that we must use the request lib to resolve those ones, I made a small change to be able to resolve this promise and return the stream to be managed by the user following his needs.

[pnedelko]

Thanks for the PR! Could you please also add some documentation/examples, because I find this feature really interesting.

[pnedelko]

Please rewrite to normal function, since this lib is not using latest JavaScript standard. We will introduce Babel or whatever in future.

[castanhob]

Sure @pnedelko !

When we attach to the Marathon Event Bus (from the Nodejs perspective) we receive a Readable Stream object.
This object can be used to see what Marathon is doing through the events. It has a big potential for DevOps to control the flux of their deployments, for example.

Here is an example on the user side on how to implement it using the marathon-node lib:

const marathon = require('marathon-node')(MARATHON_URL, opts);

marathon.events.attach()
	.then(function(stream) {
		// Forces the stream to receive a String instead of a Buffer object
		stream.setEncoding('utf-8');

		// Event that receives data from Marathon
		stream.on('data', function(chunk) {
			// Printing the event received from the stream
			console.log(chunk);
		});

		// Last event, it runs when the connection is closed
		stream.on('end', function() {
			// Here you do what you need when it ends...
		});

		// If for some reason we receive an error while connected, we can handle it here
		stream.on('errror', function(err) {
			// Error handling...
		})
	}).catch(function(err) {
		// Any problem while trying to connect to /v2/events
	});

Any further explanation about this feature or documentation that I missed, please let me know!

[pnedelko]

@castanhob could you please add this code example to readme file?

[castanhob]

Done. Could you please check if it fits the README properly?

[pnedelko]

Well done! Thank you!