Commands now return void; block console access by default

This change changes the return type of command handlers to void, given
that we shouldn't care about that return value anyway. The
CommandManager.onCommand() method will now return whether it could route
a command.

Furthermore, @CommandHandler annotations got an additional parameter,
"console", which toggles whether the command is accessible from the
console. This defaults to false, since it often doesn't make sense to
run them from the console.