Solution for #2128 -  Sort missions list

Although code is simple, please review carefully because this is my very first Lua coding. Some code issues can be introduced :p

Example screenshots:

By reward:
![ByReward](https://f.cloud.github.com/assets/1286157/325400/a16cdcc8-9b09-11e2-8419-d1593d918ceb.PNG)

By deadline:
![byDate](https://f.cloud.github.com/assets/1286157/325407/c3b8ab86-9b09-11e2-9f3b-882d33cb9f9b.PNG)