allow changing the drain property in the receiver

[jfromaniello]

Rhea supports changing the drain property to true as shown in this example:

https://github.com/amqp/rhea/blob/master/examples/drain.js#L37

[amarzavery]

Thanks for the PR. Can you let me know how do you plan to use the drain property? Some context would help improve my knowledge as well :)

[amarzavery]

0.1.8 version of rhea-promise has been published to npm.

[jfromaniello]

I was trying to follow this example:
https://github.com/amqp/rhea/blob/master/examples/drain.js

To be able to subscribe to the drain event. To be honest I am not entirely sure if this is what I want.

I've a receiver, receiving and processing jobs. On SIGTERM I want the receiver to stop receiving new messages and when it finish processing what it has then, close the connection and terminate