Handle Related Objects of Observables

[rpiazza]

In STIX 1.x, observable objects can be related to other object which appear in other Observables.

In general, relationships between cyber observables are not supported in STIX 2.0, but there are some cases (domain_name related to an ip_address via the "resolved_to" relationship) that we might want to not just ignore

[clenk]

+1 for properly converting domain_name -> resolved_to ->ip_address .

[rpiazza]

I have added the general mechanism to deal with related objects, but it only works for patterns and only for attachments and resolved-to relationships. See #97.

[rpiazza]

Need to do this for all other relationships. Only a few can be represented in STIX 2.0