Make Dockerfile checkout collins instead of ADD .

Since the tumblr team doesn't want to have the Dockerfile at / and
Dockerfiles have no access to parent directories due to security
reasons, we need to RUN git checkout instead of ADD.
Rename DOCKER.md -> README.md
Update README
Open sourcing collins-auth
Addressing comments from Dallas
More fixes
Make it nil by default
Fixes
Consistent behaviour
merge options
Fix for prompt
Merge pull request #141 from funzoneq/arnoud-collins_auth

Open sourcing collins_auth
Merge pull request #139 from discordianfish/dockerize

Fix Dockerfile
Update collins_auth.rb
Merge pull request #143 from tumblr/dallas-collins-auth-prompt

fix collins-auth prompt