Check @condition of CybOX properties nested within STIX Indicators

Attempt to fix #3 

@bworrell, this worked for the cases I tested, but your knowledge of the schemas is definitely better than mine, so I'd suggest checking the XPath I wrote to make sure I didn't miss anything. (For example, I wasn't sure if `cybox:Object / DomainSpecificObjectProperties` works the same way as `cybox:ObjectPropertiesType` based on the information in the schema, so I left it out of the XPath.)

Also, let me know if I am way off base with this.