Fire ServerCommandEvent for command block cmds. Addresses BUKKIT-3165

https://bukkit.atlassian.net/browse/BUKKIT-3165

Fires a ServerCommandEvent when a command block triggers. Currently, there is no way to catch such a command by an event. This small change triggers a very simple command with nearly no internal logic of its own (ServerCommandEvent) for these events, allowing not only detection, but modification, of the command block commands.