Invaders falling

Hey I think I've made some progress. I really hope we base future things on this.

Read the commit messages for details.

Here are some screens:

![screen shot 2014-05-16 at 04 38 38](https://cloud.githubusercontent.com/assets/506129/2992763/f65fb506-dc9a-11e3-98ab-28f13bffd73b.png)
![screen shot 2014-05-16 at 04 38 27](https://cloud.githubusercontent.com/assets/506129/2992761/f65ed69a-dc9a-11e3-91ef-8c743db77040.png)
![screen shot 2014-05-16 at 04 38 23](https://cloud.githubusercontent.com/assets/506129/2992762/f65f532c-dc9a-11e3-87bf-510fce06231e.png)
![screen shot 2014-05-16 at 04 38 11](https://cloud.githubusercontent.com/assets/506129/2992764/f65fe422-dc9a-11e3-95b1-20365561ab0c.png)