Winter is coming

CIB's overlay PR #8993 inspired me to add support for seasonal tiles.

New tileset IDs (without spaces, github was formatting underscores out without them, thanks git...): 
* {ITEM ID} _ season _ {SEASON}

Valid seasons: spring, summer, autumn, winter

You can use it for
* changing terrain depending on season
* fruit trees and berries can have tiles with fruit shown only in their harvest season
* flowers can be shown as withered / 'hidden' during winter
* mobs can have different look depending on season, like jackrabbit in screenshot below - his description is "The snowshoe hare, a small grass-eater with a brown fur coat that changes to white in the wintertime.", now it's true
* probably other things

Screenshot from game (warning, it does show lack of art skills on my side):
![winteriscoming01](https://cloud.githubusercontent.com/assets/6969268/4375641/f6eff3aa-433d-11e4-9428-f2424d7559a1.PNG)

There is WinterTest tileset included, for testing purposes only. Just a few of winter tiles were added.