[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.
I see - I think @nchammas removed these progress bars in https://github.com/apache/spark/commit/19f61c165932059e7ce156da2c71429fa8dc27f0 for some other cases. I would actually like to have the progress bars because otherwise it's very confusing what is happening when developing locally. However, we can check whether `AMPLAB_JENKINS` is set, and not show them in that case. Similar to here:

https://github.com/apache/spark/blob/daaca14c16dc2c1abc98f15ab8c6f7c14761b627/dev/run-tests#L80

If you wanted to fix the other instance as well @brennonyork, that would be great.