Spree, God items and bosses more prominent, exp adjustment

- Killing sprees are now tracked as stats.  Type "spree" to see how many
kills in a row you have without dieing, and it will also show you your
best spree.
- Players now take more damage as they rack up kills without dieing.
The current formula is 25% damage increase taken for every 100 kills in
a spree.
- God items are now shown in green in the chat so that you will
immediately be able to recognize that they have dropped
- Boss monsters now have a yellow particle trail, so the player can see
them better
- Exp has been adjusted from level 300 and onward so that you should be
able to get roughly the same exp/time no matter what level you are.  My
ideal goal is around 5 character levels per 15 minute map.