Client warning on significant time difference

Possible solution to #950
(Also see PR #1039)


Notifies the client when a significant time difference exists between the server and client (>5 seconds).

![jabbrclienttimeerrormessage](https://f.cloud.github.com/assets/1687386/1435618/5565eca2-4144-11e3-80eb-c6181145e235.png)