add Dockerfile and supporting scripts

This allows running Metis (in a dev config for now) by doing:

`docker build -t vip/metis .`
`docker run -d -p 4000:4000 vip/metis`

Then you can hit it at http://localhost:4000/ (or
http://localdocker:4000/ with docker-osx).
update README.md with docker instructions
install build-essential in Dockerfile

Some node modules need make and compilers.
install node reps in Dockerfile so they can be cached

This speeds up subsequent docker builds if the package.json doesn't change.
add node_modules/.bin to docker/run-tests.sh so it can find karma

Also cd's to /metis to run the tests from there.
setup test suite to use phantomjs & install it in Docker

Instructions for running tests in Docker added to README. Lots of failures, but it seems that was already the case.
use quay.io base docker repo
Merge branch 'master' into docker
update README Docker section
Merge pull request #217 from votinginfoproject/docker

add Dockerfile and supporting infrastructure