Allow non-root USER

Add sti-fake-user Dockerfile which extends sti-fake and runs as a
non-root user.

Add integration test to ensure that non-root images work ok.

Fix issues with support for non-root users:
- set config.User prior to committing if the base image has a USER
- workaround issue in Docker where you can't run a container from an
  image whose original container had a file bind mount, and you don't
  specify something to satisfy that file bind mount when running.
Merge pull request #48 from ncdc/non-root-user

Allow non-root USER
Add parameter to STI build to invoke an optional callback url
Merge pull request #44 from derekwaynecarr/add_callback_to_build

Add parameter to STI build to invoke an optional callback url
Add test script to pull test images
Merge pull request #49 from pmorie/test

Add test script to pull test images