MiniMaven(cmdline): offer debug output triggered by minimaven.debug=true

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MiniMaven: offer to install aggregate projects (with dependencies)

This allows us to build Fiji with MiniMaven (and without Fiji Build).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Make MiniMaven more consistent with the imagej-maven-plugin

When the imagej.app.directory property is set, the imagej-maven-plugin
creates the directory if necessary.

While at it, restructure the source code to reflect the fact that we do
not necessarily require a project with "jar" packaging, by renaming the
API's function to buildAndInstall().

Also, offer a method to install to a given directory without setting a
system property to the path of said directory first.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MiniMaven(cmdline): make usage nicer

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MiniMaven(cmdline): document 'jar' subcommand

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
MiniMaven(cmdline): add the 'install' subcommand

This builds the project and its dependencies, and installs
corresponding .jar files into the ImageJ.app/ directory specified
via the imagej.app.directory property.

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