Adds an AWS CloudTrail LogRecordIterator for reading logs

The `Aws\CloudTrail\LogRecordIterator` class allows you to iterate over log records from the log files stored in Amazon S3 that are generated by the AWS CloudTrail service.

#### Usage Example

```php
$records = LogRecordIterator::fromTrail($cloudTrailClient, $s3Client, array(
    'account_id' => '############',
    'log_region' => 'us-east-1',
    'start_date' => '-1 week',
));
// OR
$records = new LogRecordIterator($s3Client, 'BUCKET_NAME', array(
    'account_id' => '############',
    'log_region' => 'us-east-1',
    'start_date' => '-1 week',
));

foreach ($records as $record) {
    printf(
        "The %s operation for %s was executed at %s.\n",
        $record['eventName'],
        $record['eventSource'],
        $record['eventTime']
    );
}
```

##### Notes
* Other options include: `'trail_name'`, `'end_date'`, and `'key_prefix'`
* If `'trail_name'` is not provided, it is assumed to be "Default", which is what the AWS Console creates.

##### Example Output
```
...
The CreateTrail operation for cloudtrail.amazonaws.com was executed at 2013-11-22T19:19:16Z.
The StartLogging operation for cloudtrail.amazonaws.com was executed at 2013-11-22T19:19:16Z.
The DescribeTrails operation for cloudtrail.amazonaws.com was executed at 2013-11-22T19:19:15Z.
The DescribeInstances operation for ec2.amazonaws.com was executed at 2013-11-22T19:22:17Z.
The GetSessionToken operation for sts.amazonaws.com was executed at 2013-11-22T19:28:29Z.
....
```