Add null checks, making colouring say and death toggleable, refactor commands into separate class files, and add PluginMetrics support

Fixes issue #34 and adds option to change whether say and death messages are coloured. 

Also cleans up `onCommand()` by putting each command into a separate class file
This also adds PluginMetrics to collect stats for the plugin. Note, metrics haven't been sent to MCStats.org since I had metrics disabled when testing the plugin.