Tools: Add script to grab ES version for BWC tests.

This should work regardless of the platform.  Even better would be to change the BWC tests to automatically download the version if it doesn't exist (this script will just skip downloading if the version already exists).  If it could be run as a pretask whenever -Dtests.bwc.version=X.Y.Z is passed, this could eliminate manual work necessary for running bwc tests, but I'm not sure how to do that (or if it is even possible) in maven.