Make script generation script a shell script

Now that our ImageJ installation has everything included within it to
communicate with OMERO, we can generate scripts using a shell script
that simply invokes the ImageJ launcher with the script generator as
main class.
README.md: update to new installation instructions
install-imagej: make ImageJ folder configurable
ScriptGenerator: use base OMERO dir as arg

Previously, the script generator accepted the exact path (e.g.,
$OMERO_DIR/lib/scripts/imagej where OMERO script stubs were supposed
to go). But it makes more sense to accept the root directory of the
OMERO installation, verify that the argument is in fact valid, and
then generate the scripts in the appropriate subdirectory always.

With this change, the generator now exits with a non-zero error code
if something goes wrong.
install-imagej: mark script executable
install-imagej: tweak indentation
install-imagej: simplify OMERO existence check
install-imagej: tweak usage instructions

The "--all" flag can be used to generate OMERO scripts
for *all* ImageJ modules, not just headless ones.
install-imagej: tweak section header
install-imagej: be more verbose about IMAGEJ_HOME

Otherwise, the user might be mystified why a particular directory
is being selected for installation.
install-imagej: lean on the gen-scripts script

This script will be (hopefully) distributed as part of the OMERO-5.0
update site, so will be present after ImageJ is bootstrapped. Let's
just use it directly, instead of replicating any logic in this script.
WIP: how to make the installation better