an update for TiledTileset among other things

Hello! o/

I noticed that in order to get the size of an image we have to load the image (with pygame or SFML) and then get its size property, but I noticed that right now TMX format saves the image size along with his source:

```
 <tileset firstgid="1" name="terrain_atlas" tilewidth="32" tileheight="32">
  <image source="../maps/tilesets/terrain_atlas.png" width="1024" height="1024"/>
 </tileset>
```

So, I decided to add this changes to TiledTileset class. I also move the ```import pygame``` stuff because not everybody uses pygame to make games these days!

cheers!