Allowing Users to Fetch EC2 Instances in Batches

Allow users to specify 'nextToken', and 'maxResults' to fetch EC2 instances in batches. Similar to how [describe_instance_status](https://github.com/fog/fog/blob/master/lib/fog/aws/requests/compute/describe_instance_status.rb#L11-L20) is currently working
Could you add a quick comment here explaining `MaxResults` also? Thanks!