Retrieve jars to a flat directory so * can be used for the classpath.

This is somewhat trivial, but when doing "ps aux | grep java" to see what Spark processes were running, I noticed the classpath line is incredibly long.

Since JVM 6, the classpath arg can take "dir/*" and load all of the jars in that directory, which results in a much shorter command line in the ps output.

"dir/*" is not recursive though, so it requires changing Ivy to retrieve the jars in a flat manner.

I think this is a improvement, granted its minor, so understand if you're fine with the way things work now.