[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.
Would it be overkill to use a command-line utility like [`xmlstarlet`](http://en.wikipedia.org/wiki/XMLStarlet) to select this field? I'm worried that seemingly-innocuous changes to the POM might break the Maven build.