Add waiters for ec2 instance status checks

* instance-status
* system-status

... because Windows instances take way longer to come up than non-Windows.
Feedback
Merge branch 'master' into ec2_status_watchers
Moved S3 waiter customization to appropriate location.
Merge branch 'ec2_status_watchers' of https://github.com/petemounce/aws-sdk-core-ruby into petemounce-ec2_status_watchers
Moved S3 waiter customization to appropriate location.
Merge branch 'petemounce-ec2_status_watchers'

Conflicts:
	aws-sdk-core/apis/EC2.waiters.json
Renamed the EC2 instance status waiters from *Green to *Ok to match api values.
Added changelog entries for the new waiters.