Separate out FITS env; add Nailgun helper

This commit 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.

My main motivation was to create a new script which launches a [Nailgun](http://martiansoftware.com/nailgun/) server using the FITS classpath without code duplication; this commit also introduces the Nailgun script. Nailgun is a server which launches a persistent JVM that's available for Java programs. In environments where FITS is being launched repeatedly, this saves 1+ seconds of JVM startup time on every execution, which can really add up over time.