This improves Docker support in the following ways:

* Base image off `ruby:onbuild` to avoid unnecessary custom steps and ease
derived images.
* Adds `.dockerignore` to avoid sending large files to build.
* Adds a `docker-compose.yml` so users can simply type `docker-compose up`
to get it running.
* Explicity sets the volume definition in the `Dockerfile`.
* Volume is automatically mount by `docker-compose` (the `source` directory
relative to the project's dir).
* Workaround watcher failing to pick directory changes probably because of
vboxsf and the `listen` gem failing to register filesystem changes.
* Updates documentation.

Update README URLs based on HTTP redirects

Added Invoiced to list of examples on README

Use base ruby image in Dockerfile & fix watcher

It seems that the watcher will not work without the
--force-polling option.

upgraded to wily to remove error from version trusty

Basic Vagrant support. Close #348

More memory needed for `rake publish`

Need git for `rake publish`

Add hosting.de API docs to the list

make middleman pick up changes when run in docker

Add SafetyCulture API to the list

Fix Dockerfile by installing nodejs

Add SocialRadar to the list of docs sites using Slate

Use HTTPS for github urls.

Update gems

Remove therubyracer in favor of system JS runtimes

adding supporkit docs as an example

Add Aircall.io to README

Improved the appearance of code blocks when printing.