Using existing download_file.py from custom maven_jar Skylark rule
gives us the following advantages:
* fetching of sources (needed for GWT JSNI build and IDE integration)
* server support for proxies
* global, per-user cache of artifacts
* support for local maven repository (~/.m2)
Change-Id: Ic246dd9298775a6819f05453c601c688d8cb7ab9