Improve code's readability.
Extracting container interface.
Renamed interfaces, classes, and variables per bernard's advice.
Merge branch 'master' of github.com:soundcloud/harpoon into logging-api

Conflicts:
	harpoon-agent/registry.go
Renamings per review.
Merge remote-tracking branch 'origin/remove-docker-system-import' into logging-api
Simplified as per Bernards review comments.
Merge branch 'master' of github.com:soundcloud/harpoon into logging-api

Conflicts:
	harpoon-agent/container.go
Merge branch 'master' of github.com:soundcloud/harpoon into logging-api
Merge branch 'logging-api' of github.com:soundcloud/harpoon into logging-api
Merge pull request #21 from soundcloud/logging-api

agent: add Container interface + testing