The class now overwrites the constructor of the basic class to set a default
field_size value of 15 pixels.
It also returns self in the create method corresponding to the base class.
Plus I've added the documentation.
At the moment the image is simply shown to the user. But we need something
better. The user must be able to decide, what happens.
Also I want support for other colors.
Dunno why there is no earlier commit...
Map class is probably a little bit too static. I will rewort this later, but
first we'll finish some of the other stuff.