Merge release branch for v1.2

Summary of changes:
- Docker Remote API support
- addFile fixes
- Base image for Java 8 and fixes
- External Dockerfile support
- Gradle 2.0 compatibility
- Image tag version and fixes
README.md: added links to jCenter and MavenCentral
README.md: list supported Dockerfile instructions
README.md: typo
Merge README.md changes from branch 'dev'
Fixes gradle task in simple example command line
Adds support for setting user name or UID to use when running the image and for any following RUN directives.
https://docs.docker.com/reference/builder/#user
Cleanup.
Bumping version number.
Example README typo.
Merge branch 'fwaibel-master' into dev