Add the `osx-bootstrap::postgresql` recipe
Add the `osx-bootstrap::machine` recipe
Enable lookups by MAC address in the `osx-bootstrap::machine` recipe
Introduce the `OsX::Bootstrap::Ssh` library and use it, instead of `ssh-keygen`, to generate SSH public key file contents from private keys
Install Homebrew's `openssh` package to replace OS X's pre-installed, outdated version