Add missions

There are missions and then there are plots. Each mission should have
one plot for each team.

Things we still need to do:
* Testing nontrivial cases
* Styling
* Better admin page
* Show only revealed missions (time? flag? both?)

Issue #101, y'all.


![zombie_mission_list](https://f.cloud.github.com/assets/887446/185589/08d5489e-7d0c-11e2-8dab-e4a6dfe4edb5.png)
![human_mission_list](https://f.cloud.github.com/assets/887446/185590/0af74ef6-7d0c-11e2-838d-524688030a2b.png)