Add 'classifier' property to Gradle plugin

The default behaviour doesn't change with this commit, but now
the user has the option to specify a 'classifier' property
either in springBoot { classifier = 'exec' } (i.e. globally
for all repackage tasks) or in each repackage task, e.g.
bootRepackage { classifier = 'exec' }. In that case the original
archive is not overwritten but copied into <file>-<classifier>.jar
(or .war etc.) and then enhanced.

Fixes gh-1113, fixes gh-141 also I believe.