Multiple (random) log in messages

Basically add support for random log in messages because I wanted them for my server. The same message all the time gets old, and although i could have written a script to change the config every day I think this is a better solution :D.

Old config example:
```yml
timeThresholdHours: 0
chatColor: DARK_AQUA
message: The last player only logged off $TIME ago.

```

New config example with one message, using the auto-converting from the old version:
```yml
timeThresholdHours: 6
messages:
- The last player only logged off $TIME ago.
chatColor: DARK_AQUA
```

New config example two messages:
```yml
timeThresholdHours: 6
messages:
- The last player only logged off $TIME ago.
- "example of a complicated message that ' includes, random($&@#((==- chars, last player was $LASTPLAYER, $TIME ago."
chatColor: DARK_AQUA
```

Note that the randomness is even, eg each message is equally likely to be chosen.

Also because it bugged my log parsing tool I changed your logging stuff to use the per-plugin logging methods as recommended by bukkit, if that isnt ok with you feel free to revert those (or ask me to).