Implement done() - terminates a Promise chain, rethrowing unhandled rejections as Errors

[johnyanarella]

Implement done() and add to README.md.

One of the pitfalls of interacting with Promise-based APIs is the tendency for important errors to be silently swallowed unless an explicit rejection handler is specified.

For example:

promise
  .then( function () {
    // logic in your callback throws an error
    // and it is interpreted as a rejection.
    throw new Error('Boom!');
  });

// The error is silently swallowed.

This problem can be addressed by terminating the Promise chain with the done() method:

promise
  .then( function () {
    // logic in your callback throws an error
    // and it is interpreted as a rejection.
    throw new Error('Boom!');
  })
  .done();

// The error is thrown on the next tick of the event loop.

The done() method ensures that any unhandled rejections are rethrown as Errors.