Aggregate json annotations in uber-jar

[JES: attached the exec:java phase to the lifecycle to make sure that
the annotation combiner is run; also changed the package to avoid the
deprecated annotation combiner class; while at it, avoid writing to
src/assembly/all/ and instead write to target/assembly/all/]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Remove obsolete annotation aggregation readme
Use the shiny new META-INF combiner for uber jar files

We need to combine more than just the annotation indexes: script engines
implementing the javax.scripting interface need to be declared in
META-INF/services/ for example.

Hence we introduced the META-INF combiner in SciJava-commons whose job
it is to know all the things that need to be combined and just do that.

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