Add Weather System

This pull request adds a basic, working weather system.  For testing purposes, I have also included several test commands: toggledownfall, setchunkbiomesnow (quite unstable; sets the biome of the current chunk to a snowy one), and biome.  Rain and thunder times have also been tweaked for testing purposes.

There is still work to be done, but this feels like a good solution.

As a side note, sorry for breaking line-endings.  I tried every option my IDE gave me, and none of them worked.