Check jarsigner is executable
Log proxy args and directory to be zipped. Warn about password logging.
release-webstart build target replaced with signed/unsigned targets

Removed release-webstart build target
Add release-webstart-unsigned target (doesn't sign jars)
Add release-webstart-signed target (self signs jars using omero_insight_sign.py).
release-all runs release-webstart-unsigned so the final artifacts will be unsigned.

webstart-sign target has been changed to use the omero_insight_sign.py script instead of the old jarsigner ant calls