So marking functions like `add_markings()` can be directly called as methods on
SDO/SRO classes. Note that they return new versions of the objects
because objects are immutable.
- Typos in Attack Pattern tests
- Put MarkingDefinition, ExternalReference, and KillChainPhase together
in a file for objects that aren't SDOs or SROs
- Create utility function to return dictionary from string or
file-like object
- Put off testing parsing Cyber Observable Objects until a later commit
- improved timestamp formatting
- python-stix2 will only include subsecond values if they don't equal 0
- in Python 3.6, datetime.astimezone doesn't throw an error on naive
timestamps as in previous versions
- Update ReferenceProperty to allow specifying a particular object type
- Update ListProperty and add SelectorProperty
- Add description to Relationship
Chris Lenk