Reorganizing spec directory
Rename `docker rebuild` to `docker rebase`
Not sure how this ended up back - git rebase!!!!
Refactor `docker build`

Added and modified two mixins: Chef::Knife::ContainerBase and Chef::Knife::ContainerDockerBase.
Refactor `docker init`

* Leverage common attributes shared by Chef::Knife::KnifeContainerDockerBase.
* Replace `setup_default_config` with `reconfigure` method, make the rest private.
* The base image value in knife.rb should now be `knife[:base_docker_image]`.
Refactor `docker rebase`
Refactor Berkshelf::Berksfile integration

* added spec tests
* added PluginError for when a plugin errors out
Add specs and docs for Plugins::Berkshelf
Add docs and specs to Plugins::Docker::Context
Add specs and docs for Plugins::Docker::Image
Add docs and spec to Plugins::Docker