Fixes gh-611, gh-612 - Add kill count system and kill meters with XP.

The killstreak counter shows a message if the player has more than 5
kills. I have also added an example reward system with points, but did not
enable it yet (needs discussion).

Through the warzone option xpkillmeter the XP level will get updated with
your current kill streak amount. This gets reset every time you die and
is added to every time you kill a player. If this option is enabled,
players cannot gain XP in the warzone.