Abort unhandledRejection when Promise does not handle exceptions.

[yosuke-furukawa]

Current Node process does not abort on unhandledRejection.
So if developers write the following sample test code,

const assert = require('assert');

function testRejectPromise() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      reject(new Error('test'));
    }, 100);
  });
}

testRejectPromise().then(() => {
  assert('test');
});

eater cannot catch the error.

workaround is here.

const assert = require('power-assert');

function testRejectPromise() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      reject(new Error('test'));
    }, 100);
  });
}

testRejectPromise().then(() => {
  assert('test');
});

process.on('unhandledRejection', (e) => {
   console.error(e);
});

After merging this PR, eater can handle unhandledRejection when promise reject an exception.

[coveralls]

Coverage remained the same at 100.0% when pulling 939f83c on add_promise_unhandled_rejection into 88643e4 on master.

[yosuke-furukawa]

if this issue is solved, I can revert this commit :)
nodejs/promises#26