Added a new configuration option, Spawn Chance, which allows you to configure the percent chance of an entity spawning every 1/20th of a second. Similar to when configuring the amount of entities per wave, you can use formulas with this config option. Full explanation is given in the actual config file.
Preliminary work on support for an Achievements plugin. The Achievements plugin has not yet been released, so you can't actually do anything with that yet.
