Improve CRITs sharing via standards

*These changes are intended in conjunction with pull requests I have (or am about to) issue to `crits/crits_dependencies` and `crits/crits_services` with identical branch names.*

This may need some clean up, but functionality is here, so I wanted to get it PR'ed so review can start.

The changes I have made to the TAXII Service required some alteration of CRITs core source.

Highlights:
- Create `to_stix` method on base CRITs mixin that handles generating a STIX document for the crits object
- Add `to_cybox` code for `domains`, `emails`, `IPs`, `samples`, and `PCAPs`
- Add methods to said objects (and `indicators`) for providing information for `STIXHeader` when appropriate
- Update the `object_mapper` to convert more `indicator` types to available `CybOX` objects.

Regarding that last point, we need to review the **entire** large list and decide which items to drop (as there may not be a good or uniform way for us to put the limited data in the CRITs indicator (the `value`) into complex objects provided by `CybOX`) or to modify. In `object_mapper.py`, items that seem not to have any relevant `CybOX` object available have a `# TODO cybox-unavailable` comment. Items that have an object match that I'm not sure if/how to map have a `# TODO REPR` comment.

**Note:** we should DEFINITELY test to make sure that the upgrade of STIX and CybOX to the newest releases doesn't break other functionality in CRITs that depends on these.
The issue we are seeing now is that sources aren't being appended appropriately. If I have a Sample with a source of "foo", I export it as a STIX doc, and then use STIX Import on it setting a source of "foo", it does not add a new *instance* of "foo", it adds an entirely new entry for "foo".