$ec2->waitUntilSshIsActive()

Here's a first pass that is ready for feedback, but not yet ready to merge. I need to get a better understanding for:

1. How to hook up this custom waiter internally in the SDK (not currently documented)
2. Since this requires integration tests which hit an actual EC2 instance, I couldn't find any tests that were useful for reference. Also, it isn't clear how you handle unit vs. integration tests in the SDK.