Verify download url by HEAD request first

Do a HEAD request using httplib2 first, to see if the requested
runtime version is available for download at all. This could lead to a
bit more understandable error message instead of garbage from wget and
tar on the screen.

httplib2 does follow HTTP redirects etc. by default
magically prepend runtimes in app_base...

...to mxjar_repo, as already stated in the documentation
(full-documented-m2ee.yaml)
Update CHANGES, NEWS about download_runtime