#515 Improve derby support

[Dave Dyer]

I recently downloaded squirrel explicitly to use with a derby embedded database. It took a little detective work to get it running, because the derby jars were not included in the distribution, and nothing I encountered in the setup instructions alterted me to the fact that they were needed.

I suggest the easiest way to fix this would be to include the jars. If that's not feasable, then at least when I attempt to use the derby driver, and the jar is not found, an explicit and helpful message could be presented, rather than the generic failure message.