Add ec2 instance state reason

- Add instance.reason and instance.state_reason
(http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-Item
Type-StateReasonType.html)
- Add ec2 filtering by state-reason-code and state-reason-message
Test filtering get_instances by reason code
Merge pull request #234 from DataDog/add_instance_reason

EC2: Add instance state reason