New class DirtBaseGenerationLayer + tests

Added a dirt base generation layer that spawns a square of dirt tiles
and tested it on the main.

Also added a "return null;" on Gallimimus.as because it was throwing a "Function doesn't return" error.

16 with and without beachLayer:
![screen shot 2013-11-22 at 17 27 32](https://f.cloud.github.com/assets/3168881/1605557/b36b29e6-53de-11e3-9ac5-074f85c8689f.png)
![screen shot 2013-11-22 at 17 28 53](https://f.cloud.github.com/assets/3168881/1605558/b37c64e0-53de-11e3-92fd-3b9d7d7b5045.png)


Resolutions: 
64
![screen shot 2013-11-22 at 17 15 57](https://f.cloud.github.com/assets/3168881/1605519/dc5edce6-53dc-11e3-9337-377a206a1220.png)

16
![screen shot 2013-11-22 at 17 16 59](https://f.cloud.github.com/assets/3168881/1605523/2dbd093c-53dd-11e3-91f8-7127a3e9aec7.png)

8
![screen shot 2013-11-22 at 17 17 20](https://f.cloud.github.com/assets/3168881/1605524/375b13f8-53dd-11e3-9c22-e45f67f3a5bb.png)

4
![screen shot 2013-11-22 at 17 17 34](https://f.cloud.github.com/assets/3168881/1605527/3b418394-53dd-11e3-8616-44a434529513.png)

2
![screen shot 2013-11-22 at 17 17 55](https://f.cloud.github.com/assets/3168881/1605528/40fd525e-53dd-11e3-9bb9-756c4c502dc8.png)

1
![screen shot 2013-11-22 at 17 18 15](https://f.cloud.github.com/assets/3168881/1605529/44a66e22-53dd-11e3-81fc-0f08ac56b3a8.png)