[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.
If I remove the `-s` flag it would show the download progress, but, as @ScrapCodes mentioned, if we leave it in then it would pummel the Jenkins logs with said download progress. @pwendell your call if you'd rather see it in or not, just let me know!