POM: start version at 0.1.0-SNAPSHOT

Let's not recommend to everything that they a non-incubation release
version by default for their Maven projects. Let's recommend they use
a SNAPSHOT, versioned in the incubation phase according to SemVer.
POM: bump to pom-imagej 5.0

This purges any tainted references to the
not-quite-successfully-released pom-imagej 3.2.
POM: use inherited exec profile for main class

We just set the main-class property, and then
"mvn -Pexec" should magically work to launch it.
POM: remove redundant configuration blocks

Let's not make it _too_ easy for people to override Maven's very
sensible defaults with something less sensible -- even if there are
times when you need to do it. Those times should be the exception
to the rule, and those people need to learn a little bit more about
Maven in that case.