Do not pollute the local home directory from the test suite

Remove the `cp -a * ~/` from the custom service buildscript.
Import the actual list of valid services
Actually check if services are buildable or not (closes #16)
Add Container.stop to interrupt a running container

Using `docker stop`.
Add Service.stop to interrupt a building or running service
Catch SIGINT/SIGTERM in `sandbox run` (closes #17)
Fix the error shown when a command doesn't return 0 in the builder
Improve the errors displayed when a Docker image is missing
Add the attribute Container.exit_status (see #18)
Use Container.exit_status to check if the build went fine (closes #18)