Update to a better release process

This imitates the process developed in the ImageJ project.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Revert "Do not use the GPG agent for the release"

Actually, it *is* a good idea ;-)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge remote-tracking branch 'origin/master'

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Bump to next development cycle

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>