Don't register HUP signal handler on Windows

Don't register HUP signal handler on Windows.

Fixes bug #244

```
$ bin/riemann etc/riemann.config
INFO [2013-06-20 09:08:13,593] main - riemann.bin - PID 277056

ERROR [2013-06-20 09:08:13,594] main - riemann.bin - Couldn't start
java.lang.IllegalArgumentException: Unknown signal: HUP
        at sun.misc.Signal.<init>(Unknown Source)
        at riemann.bin$handle_signals.invoke(bin.clj:36)
        at riemann.bin$_main.doInvoke(bin.clj:58)
        at clojure.lang.RestFn.applyTo(RestFn.java:137)
        at riemann.bin.main(Unknown Source)
```