[SPARK-4501][Core] - Create build/mvn to automatically download maven/zinc/scalac

Creates a top level directory script (as `build/mvn`) to automatically download zinc and the specific version of scala used to easily build spark. This will also download and install maven if the user doesn't already have it and all packages are hosted under the `build/` directory. Tested on both Linux and OSX OS's and both work. All commands pass through to the maven binary so it acts exactly as a traditional maven call would.
Actually I had originally used `python` to grab this out in a more concrete way, but @pwendell recommended we remove as many dependencies as possible so I switched over to the `grep | cut` scenario. Let me think it over and I might be able to find `bash`-only way to assuredly grab the correct version. That said, we could always add an entry to `project/build.properties` (e.g. `scala.version=2.10.4`) and assuredly pull it from there without all this hassle. What do you guys think?