Add method to access private IP address of instance.
Add spec for private IP.
Use public_hostname by default unless overridden.
Allow connection to decide public or private unless a string host is
explicitly passed.
Decouple hostname from live check.
Use ip automatically if public hostname is nil.
Add a spec for vpc connections
Merge pull request #18 from swipely/vpc_private_subnet_support

Vpc private subnet support