how do i specify the data directory?

```
% ls mydir
ls: cannot access mydir: No such file or directory
% ./bin/pulsed -port 8080 -addr localhost:5000 -dir mydir
2014-12-06 16:38:32,089 INFO  main org.eclipse.jetty.util.log:188 Logging initialized @250ms
2014-12-06 16:38:32,131 ERROR main com.github.zk1931.jzab.Zab:476 The log directory is not empty while joining.
2014-12-06 16:38:32,131 WARN  main com.github.zk1931.jzab.Zab:172 Caught an exception while initializing Zab.
Exception in thread "main" java.lang.IllegalStateException: Failed to initialize Zab.
        at com.github.zk1931.jzab.Zab.<init>(Zab.java:173)
        at com.github.zk1931.jzab.Zab.<init>(Zab.java:138)
        at com.github.zk1931.jzab.Zab.<init>(Zab.java:102)
        at com.github.zk1931.pulsed.Pulsed.<init>(Pulsed.java:60)
        at com.github.zk1931.pulsed.Main.main(Main.java:100)
Caused by: java.lang.RuntimeException: Log directory must be empty.
        at com.github.zk1931.jzab.Zab$MainThread.<init>(Zab.java:477)
        at com.github.zk1931.jzab.Zab.<init>(Zab.java:166)
        ... 4 more
```