XPath in Controlled_Structure does not work well for non-XML implementations of STIX

[terrymacdonald]

With the current likely MTI Serialization Format being JSON (with a possibility of a binary serialization protocol in the future), the current use of XPath in Controlled_Structure part of the Marking Structure does not work.

We need to find a different solution. One potential solution is proposed below:

1. Remove Handling/Marking from STIX_Package Header
2. Remove the concept of XPath Controlled_Structure
3. Find a new home for Information Source, outside of Handling/Marking
4. Create Simple Marking, TLP, and TOU marking as either attributes to each object or as elements within each object

[terrymacdonald]

This issue is related to #378 and #379.