Add valid_from and valid_to to relationship

[johnwunder]

Many relationships are only valid for some time period...for example, malware may only be used by a threat actor for some period of time. Optional valid_from and valid_to properties would let you capture that, when relevant.

[gtback]

We should be careful of how this interacts semantically with (for instance) the valid_from field on Indicators.

[JasonKeirstead]

I think that first_seen and last_seen makes more sense given the information being captured here.

[lpingree]

Agreed

[lpingree]

I think it would be nice to also have a "times seen" but the concurrency and state may be problematic.

[skelley1]

This issue was superseded by our solution to Issue #11.