Use the ProcessUtils to fall back to javac

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Document what the JavaCompiler class is all about

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Let the user know about the cause when we could not fall back to javac

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Look more closely when maven-metadata-snapshot.xml was not found

When said file is not there, the local setup might have mirrors
configured and might look for the snapshots in ImageJ's
snapshots repository. Let's handle that case.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'assorted-fixes'

This branch mainly addresses the issue described in

	https://github.com/scijava/minimaven/issues/10

as well as issues that came up while fixing it.

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