Separate out FITS env; add Nailgun helper

Separates out the FITS classpath setup, which is fairly complex, into
its own script which can be included in other scripts to set up a
compatible environment.

Additionally adds a new startup script, fits-ngserver.sh, which starts
up a Nailgun server with the FITS classpath instead of launching FITS
itself. This produces a persistent JVM to which future FITS instances
can connect, reducing startup lag over multiple iterations. For
environments where FITS may be called repeatedly over thousands of
files, saving even one or two seconds per launch can save a
significant amount of time in total.