If not configuration value set, choose sensible default, don't crash!

```
13:35:54.639 [main] INFO  com.buddycloud.mediaserver.Main - Buddycloud Media Server HTTP server started!
13:35:54.645 [main] ERROR com.buddycloud.mediaserver.Main - Error while starting XMPP client / component
java.lang.NumberFormatException: null
	at java.lang.Integer.parseInt(Integer.java:417) ~[na:1.6.0_26]
	at java.lang.Integer.valueOf(Integer.java:554) ~[na:1.6.0_26]
	at com.buddycloud.mediaserver.Main.setXMPPReplyTimeout(Main.java:115) ~[buddycloud-media-server-jar-with-dependencies.jar:na]
	at com.buddycloud.mediaserver.Main.startXMPPToolBox(Main.java:120) ~[buddycloud-media-server-jar-with-dependencies.jar:na]
	at com.buddycloud.mediaserver.Main.main(Main.java:52) ~[buddycloud-media-server-jar-with-dependencies.jar:na]
```