The winter update, part 2

![cold_weather_in_hell](https://cloud.githubusercontent.com/assets/7573912/5426651/bc7b8f48-8363-11e4-962a-a85dee5b8c3f.png)

https://dl.dropboxusercontent.com/u/33846352/dreamseeker%202014-12-13%2023-27-20-76.webm

https://dl.dropboxusercontent.com/u/33846352/dreamseeker%202014-12-13%2020-46-46-07.webm

Featuring a new random event, snowmen, snow, lots of snow based stuff and foliage, and of course christmas trees.

 * Added a new random event, Cosmic Freeze! Has an higher chance to occur in December, January and February.
 * Added new structures, snow and cosmic snow. Cosmic snow is spawned on a tile somewhere on the station when the Cosmic Freeze begin. An alert message is broadcasted.
 * Cosmic snow will rapidly propagate on tiles with an ambient air temperature of at most 23 degree Celsius. Only walls, closed airlocks, windows, and airtight plastic flaps can block its propagation. (and unsimulated turfs, like the asteroid)
 * Cosmic snow slowly lowers the ambient temperature. A small dozen of tiles won't have too much of an impact, but since they propagate quickly, the temperature might very quickly dive all the way to the minimal -40 degrees Celsius.
 * Attacking the snow with bare hands will allow you to dig up a snowball. Attacking the snow with a shovel or spade will instantly dig up 3 snowballs, additionally, digging up the snow allows you to click on objects bellow the snow.
 * When dug, the snow will slowly regenerate. It takes 40 to 80 seconds for a snow tile to become full again.
 * Added a new sheet item, snowballs. Snowball can be stacked (like wooden planks or metal sheets), thrown at people to stun them for half a second, or used to build snow fortifications and snowmen.
 * Added a new mob, Snowmen. These talkative mobs are of the "retaliate" family. Just like goats or clowns, they will attack those who anger them. However, snowmen attack by throwing snowballs. Making them mostly inoffensive. Additionally, they might get bored of fighting more or less quickly.
 * You can "defeat" a snowman in a snowball fight by throwing enough snowballs at him before he gets bored. Doing so, he will award you a random winter gift.
 * Snowmen also have a chance to offer you a gift when you equip them with a carrot. You can also give them any hat. They will drop the hat whether when you give them a new one, or when they die.
 * Snowmen, Snowballs and Snow itself have a melt temperature. When reached, the snowmen start taking heavy damage, while the snow will disappear and wet the floor in its place.
 * As the total number of cosmic snow tiles on the station increases, new cosmic snow tiles have a chance to trigger various happenings: they can spawn a sappling, which will mature after 1-2 minutes into either a dead tree or a pine tree, and above a certain number they will even trigger a migration of space bear (similar to space carp migrations).
 * Above a certain number of snow tiles, pine trees also have a chance to spawn either spiderlings or bears when they mature.
 * When pine trees mature, they have a chance to become a Christmas Tree, in which case they will spawn tons of gifts on their surrounding tiles. Hunt for the christmas trees people!
 * Dead trees and pine trees can be cut down with either a hatchet or a fire axe. The sapplings can also be removed with a mini-hoe. Removing the snow under a sappling will kill it. Removing the snow under a tree will periodically damage it, until it falls. Trees spawn a few logs when they fall.
 * If a cosmic snow spawns on a tile occupied by an Air Alarm, the air alarm will enter a frozen state. Its remote control will be deactivated, and it won't trigger any alarms. The air alarm can be unfrozen by using a lit welding tool on it.
 * Clothes now have hot breath and cold breath resistance variables. Basically, wearing items with good breath resistance will protect you from the damage usually dealt by breathing air of extreme temperatures. (scarves, balaclava, chicken head, and a few other items will protect your breath from the cold down to -70 degrees Celsius.
 * (wip)Clothes now have a cold speed protection var. Wearing items with a good cold speed protection lets you run at normal speed at lower temperatures. This is temporary until I actually make clothes warm up their users (hopefully in part 3).

these badly need to be defines but that's outside the scope of this PR (will do it in a graphics library at one point, pushing them up to 100/200/300/400 instead of decimals so they'll need to be edited then anyway)