Bump the version to a SNAPSHOT version

We are about to make changes, so let's use a SNAPSHOT version in case
Jenkins tries to deploy them.

The script used by Jenkins to upgrade the version number in the pom.xml
file will handle this gracefully:

	https://github.com/fiji/ImageJA/blob/tools/sync-with-imagej.sh#L58

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a Main-Class attribute to the manifest

That way, the built artifacts can be run using 'java -jar' directly.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Support starting the application via 'mvn exec:java'

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