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.
gen-scripts: add TODO reminder
Remove obsolete launcher scripts

We definitely do not want to be wrapping the "jython" executable with
our own hacky shell script. And the "imagej" launcher is no longer
necessary now that we are distributing all required OMERO JARs with the
OMERO-5.0 update site, and no longer need to add JARs from the actual
OMERO installation's lib/server directory to the classpath.
install-imagej: fix references to installer script
gen-scripts: always use $0/.. for ImageJ directory

This is because the gen-scripts script is expected to be distributed via
the OMERO update sites, and thus live in the lib directory of the ImageJ
installation.
gen-scripts: fix argument validity test
Add script to run a class with the ImageJ launcher

This is a generalization of the logic in gen-scripts.
gen-scripts: lean on run-class to do script gen

We will need the run-class feature for another script that calls the
script runner as well...
Add script to execute an OMERO script wrapper
ScriptGenerator: remove extra newlines