Allows to specify default and per instance layers names (fix issue #5)

Allows to specify default and per instance names for the main tile and game objects layers when creating a tile map.

Everything is back compatible, it works like before if the new selectors are not used.

With this patch you can:

set default layer names, somewhere when the application starts or before a map is loaded using those static selectors:
```
[KKTilemapNode setDefaultMainTileLayerNames:@[@"ground", @"Ground"]];
[KKTilemapNode setDefaultGameObjectsLayerNames:@[@"Objects"]];
```

or set layer names per map instance using the overloaded factory selector:
```
KKTilemapNode *map = [KKTilemapNode tilemapWithContentsOfFile:@"desert.tmx" usingMainTileLayerNamed:@"Ground" andGameObjectsLayerNamed:@"Objects"];
```