Diplomacy: Trade agreement and alien colonies

```
If the player goes into a trade agreement, the Trader fleets should travel to 
the alien planets.

In addition, the relation record should hold flags for this trade agreement. 
Not sure if the original game let an agreement succeed only once or multiple 
times?
```

Original issue reported on code.google.com by `akarn...@gmail.com` on 2 Jul 2012 at 10:27
