Debian packaging improvements

Some small changes to make the Riemann deb nicer to work with.
I'm actually not certain on this one, now that you bring it up.  There's a separate packaging manual for Java: https://www.debian.org/doc/packaging-manuals/java-policy/

[The section about java apps](https://www.debian.org/doc/packaging-manuals/java-policy/x95.html) seems to encourage the use of jarwrapper to produce executable jars, or the use of a wrapper script (as Riemann has), in which case I think the jar is effectively a private library, which it wants to put in /usr/share/<package>.