cd target/ before doing the code-signing

This is so that ./build.py clean will remove any logs or temporary directories
Disable jar-signing in update-version target
Use system proxy environment variables
Add verbose option (prints out full jarsigner command)
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
Remove sign option from jar_update_if

This means update-version won't sign any updated webstart jars.
(Actually it didn't anyway since it was removed in commit ff99355573f0145cf9eca3bed6c345b5232eded7, this just cleans up build.xml)
Remove the remaining unused code-signing ant tasks
Allow jarsigner without args to return 0 or 1

Return code depends on Java version
Change check_jarsigner error message (since exit 0|1 is allowed)

Just in case any pedants are reading