#58 thumbnail generation threading

[Joe Romaine]

In the thread of execution that generates thumbnails
(spawned from the initialization servlet), set the
priority to be lower than the rest of the web
application. Also, make that thread respond gracefully
to JVM termination requests (analaguous to SIGHUP) such
that partial thumbnails, etc. are not generated. The
executor service, or callable interface, both new to
JDK v5.x may be used to this end.