Added a simple filter system that utilizes JavaScript (Rhino)

This makes it possible to filter packet events with arbitrary code.
Arbitrary code execution is very dangerous. Limit to debug mode.

The filter command allows users with sufficient permission (or OPs) to
execute arbitrary JavaScript (no sandboxing). This is fine for a
debug and testing, but could potentially be exploited in a
production environment. 

Instead, we disable this command by default and force users to enable it
specifically in the configuration file (not through commands). If
someone has access to the config.yml file, they probably also have
access to the plugins/ folder and thus the ability to install plugins
with arbitrary code execution as well.
Minecraft 1.5.1 seems to work fine.