Get the directory watcher working again

I noticed that the basic infrastructure to support reloading the json-based config on the fly was already present, but didn't work. Dug through the code, and I believe I found the issues.

First of all, in WatchDir.java, the watcher only passed back the bare filename, instead of the full path to the file. This made the isFile() check in JmxTransformer fail to find the file. So I modified that to find the full path and go from there.

Second, the Overrides had a bunch of bugs that prevented them from properly reloading the config - fixed that up.

Third, when reloading the config, the master server list was not being cleared - so if you delete a json file, that server would still end up being monitored. Modified each watcher class to ensure that the config is wiped all the way, since the config is reloaded from scratch each time.

I've tested additions, deletions, and modifications, and they all appear to work to me. I'm not a Java coder by nature, so there could be some mistakes here.. but it's working so far for me.  :)