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

Fixes #1

I think we should possibly consider more tags on `buildpack-deps` so we could use it here too (ie, `buildpack-deps:debian-wheezy`, etc might be appropriate), but I don't know how common it is for Java things to depend on C code and/or C headers (I would guess that it's not that common).