engine.Installer: a standard interface for "installable" services

Installer is a standard interface for objects which can "install"
themselves an engine by registering handlers.

This can be used as an entrypoint for external plugins etc.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
engine: allow registering a "catchall" handler which receives all commands

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Remote communication between engines using beam

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
engine.Sender and engine.Receiver support stdin

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
engine: 'rengine' is a small command-line utility to debug remote engine

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
[Documentation - API] Add missing 'signal' parameter for /containers/:id/kill endpoint

Docker-DCO-1.1-Signed-off-by: Leo Unbekandt <leo.unbekandt@appsdeck.eu> (github: Soulou)
Added back OAuth and Accounts API docs pages

Removed a now unused endpoint from the accounts API.
Updated some of the accounts links to point to www.docker.io
as the account signup and resend-email-confirmation links should
no longer point to the index.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Merge pull request #5425 from jlhawn/io-auth-accounts-api-docs

Merged.
Merge pull request #5374 from shykes/beam-engine

Remote communication between engines using beam
Merge pull request #4576 from Soulou/4549-doc-kill-signal-param

Add missing 'signal' parameter for kill endpoint
api: added metrics to api and cli with docs

Docker-DCO-1.1-Signed-off-by: Evan Hazlett <ejhazlett@gmail.com> (github: ehazlett)