install jq (http://stedolan.github.io/jq/)

This is used to manipulate the image
config from hack/ shell scripts.
add test preparation script

this pre-builds an image with
- docker's test dependencies downloaded
- docker-test-image pulled to the right place
  for runtime_test.go

It recommits the image as docker-test, sharing
volumes but otherwise preserving config.
update the docs.

I decided to simplify the doc by pushing some of its
requirements into hack/run_tests.sh.

I think this lowers the bar for running tests.

Not sure if this is the right thing, but works
for me.