builder/virtualbox, communicator/ssh: fixes for running on windows

Jstamerj/windows

Strip carriage returns from end of lines in virtualbox/driver.go, fixing wait for virtualbox host shutdown.
Fix target upload directory path in communicator.go to use forward slashes.  (When running on windows path/filepath returns backslashes...which does not work when the target host is unix).

This adds the "amazon-instance" builder to Packer which can create
instance-storage backed AMIs. The instance-storage backed AMIs are built
by launching another instance-store backed instance, provisioning it,
and then using EC2 AMI tools to bundle and register a new AMI.