FSE-205: add Plugin library structure to knife-container

I replaced the concept of Helpers with Plugins. Integrations with APIs outside of Chef should occur in a structured Plugin. The first two plugins I have added are Berkshelf and Docker. Each plugin has the necessary substructures: Berksfile for Berkshelf; Image and Context for Docker.

I also added an Exceptions class where KnifeContainer specific exceptions will be managed.

Breaking Change: The long options for generating a berksfile has been updated from `--berksfile` to `--generate-berksfile`.