hillup-seed cant make retina size tiles

add --tile-size argument to hillup-seed.py for making different size tiles, ex 512 for retina

```
python hillup-seed.py -b 41 -121 42 -120 --tile-size 512 4 5 6 7 8 9 10
```

defaults to 256, uses tilestache tile_height layer property under the hood