Don't spawn multiple enemies on one tile

Having an enemy group defined here: (Definitely more enemies than there is space for them)
![bug1](https://f.cloud.github.com/assets/455868/496001/57e34e54-bbd7-11e2-85c6-12d3ca4aaaaa.png)
It looks like this: Multiple enemies on one tile.
![bug1a](https://f.cloud.github.com/assets/455868/496002/597d854a-bbd7-11e2-8705-2e09c273160a.png)

Resolved:
![bug1b](https://f.cloud.github.com/assets/455868/496004/5b565b3a-bbd7-11e2-8ff0-305bcb33974a.png)