Move Dockerfile to a "7" subdirectory to make way for multiple versions of Java
Add version pinning to make sure we control that closely and can make meaningful tags
Add "curl" and "wget" since they're extremely useful in dependent Dockerfiles
Add new "6" and "8" subdirectories containing OpenJDK 6 and 8 respectively
Merge pull request #2 from infosiftr/more-java

Add Java 6 and 8, pin 7 to a more specific version number, and add "curl" and "wget" for downloading dependencies in dependent Dockerfiles