Simplify GenericRelationshipLists

I tried to simplify this code a bit so we didn't keep duplicating the same file over again, only changing variable names. All of the unit tests still pass, and here's an example of using the new classes. Note that the `add_*_*` functions are gone and can be replaced with a simple `append`.

```python
from stix.common.related import RelatedThreatActor
from stix.incident import Incident
from stix.threat_actor import ThreatActor
i = Incident()
ta = ThreatActor()
ta.title = "Bob"
i.attributed_threat_actors.append(ta)
rta = RelatedThreatActor()
rta.confidence="High"
rta.threat_actor = ThreatActor()
rta.threat_actor.title = "Jim"
i.attributed_threat_actors.append(rta)

print i.to_xml()
```