New Dockerfile class (with tests).
Added appendAll and enabled method chaining.
Ensure that Dockerfile instructions are always plain Strings (e.g. not GStrings)
Use Dockerfile class, unset MAINTAINER if undef

- Use new Dockerfile class in DockerTask.buildDockerfile
- New DockerTask.buildDockerfile test & test cleanup
- Don't set MAINTAINER if undefined
- entryPoint and defaultCommand can be set as property or via method call.
Updating README.md
README.md: added info about external Dockerfiles
Preparing for 1.2: docs, version, repo.
Merge branch 'release-1.2' into dev