[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.
That would probably be less brittle, but I guess it would introduce another dependency which we'd have to install in this script (since we want it to be a one-click installer).  Since `xmlstarlet` binaries aren't portable, the installation logic might be complex since we'd need to have some platform-specific logic to download the right binaries.