Detecting nearest agent + integrating my behaviour of xml including.

1. My behaviour of xml including is tested and safe.
2. Cache.SwitchAgent is more a method, than a property. Now it is a method.
3. Added behaviour to select first agent at first start.

Algorithm for selecting first agent:
1. If we have accepted non-storyline mission, then we go to that agent.
2. If we are docked and we have an agent in current station, we will use him.
3. If we are undocked and we have an agent in current solar system, we will use him.
4. If an agent is still null, we fallback to default behaviour (selecting by Priority).