- Internal plugins are compiled into the same packer binary and invoked through the plugin command
- Search paths allow disk-based plugins to override and should function as normal
- This should allow for a 94% space savings vs statically compiling all the plugins as separate binaries.. approximately 24mb vs 431mb

Added builder plugins: builder-parallels-iso and builder-parallels-pvm.

The null builder is not really a bulider, it just setups a SSH
connection and runs the provisioners. It can be used to debug
provisioners without incurring high wait times. It does not create any
kind of image or artifact.

This closely modeled after the same provisioner
in Vagrant.

Implemented initial working version
of Docker image importing code. #774

Implement login to a docker registry, error handling

mitchellh/packer's docker branch. #774