core: make operations promise friendly

[callmehiphop]

This makes operations Promise friendly. By default they are still EventEmitter instances, but now include a .promise() method as a convenience.

bigquery.startQuery(query)
  .then(function(data) {
    var job = data[0];
    return job.promise();
  })
  .then(function(data) {
    var metadata = data[0];
  }, function(err) {
    // Something went wrong!
  });

With Bluebird

bigquery.startQuery(query)
  .spread(function(job) {
    return job.promise();
  })
  .spread(function(metadata) {
    // yay
  });

In Node >= 6.0:

bigquery.startQuery(query)
  .then([job] => job.promise())
  .then([metadata] => {}, err => {});

[stephenplusplus]

Idea and code lgtm.

[callmehiphop]

@stephenplusplus tests have been added, PTAL!