Support getting credentials asynchronous

[tellnes]

This is needed eg. to support using credentials from EC2 metadata service.

Maybe something like this:

var queue = sqs({
  // This should be called each time `sqs` needs to use the credentials
  getCredentials: function (callback) {
    callback(null, 'temporary-access-key', 'temporary-secret-key');
  },
  region:'us-east-1'
});

Or if we make it sqss jobb to now when the credentials expires:

var queue = sqs({
  // This should be only like 15 seconds before the credentials expires
  getCredentials: function (callback) {
    callback(null, { access: 'temporary-access-key', secret: 'temporary-secret-key', expires: '2014-07-13T00:36:27Z' });
  },
  region:'us-east-1'
});