added some test cases
in Observable constructor permit no _valid_refs in kwargs
in Observable._check_property ensure that the prop_name is in the kwargs
- 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
clenk