LaunchAgent for fuseki

So I've had a go at creating a LaunchAgent. I'm no expert on either fuseki or launchd so any improvements / bug fixes are welcome.

Summary of changes:

* The sample config file is copied (not installed) to `etc/fuseki.ttl`, which the LaunchAgent plist references
* Logging in all cases is now written to the filesystem
  * The log4j config is updated and installed to `etc/fuseki.log4j.properties`
  * The fuseki-server binary is modified to pass the log4j config location to the `java` command

I did want to retain the ability to log to STDOUT, but without extensively rewriting the `fuseki-server binary` I couldn't see a way. Instead I added a note in the caveats on how to tail the logs.