v0.3.0

The goal of this release is to ssh all the things in order to support any system reachable over SSH. In addition to local Vagrant machines, chef-runner can now provision remote machines like EC2 instances. To achieve this, I made the following changes:

• The argument passed to -H now has the format [user@]hostname[:port],
  allowing you to optionally change SSH user and port. (Other SSH settings can
  be set via ~/.ssh/config.)
• rsync over SSH is used to transfer files to /tmp/chef-runner on the target
  machine. chef-runner no longer depends on /vagrant being mounted.
• With Vagrant, instead of running commands via vagrant ssh, feed the output
  of vagrant ssh-config into OpenSSH. The same SSH configuration is used to
  upload files with rsync.

Other changes:

• Introduce flexible driver concept (inspired by Test Kitchen). A driver is
  responsible for running commands on and uploading files to a machine using
  whatever mechanism is available. chef-runner currently contains drivers for
  Vagrant and SSH, but more can -- and will -- be added.
• Always transfer files with rsync --compress to speed things up.
• Remove Cucumber scenarios. They didn't add much value to the Go tests and were
  very slow. Now Travis builds are much faster.
• More and better Go tests.
• More and better log messages.
• Option -h outputs more useful usage text (the one shown in the README).