Added Support for Capturing Observable Sightings

Added support for capturing Observable sightings through the following changes:

* Added a sighting_count attribute to the MeasureSourceType in CybOX Common
* Added a sighting_count attribute to the ObservableType in CybOX Core
* Changed maxOccurs of Observable_Source in ObservableType of CybOX Core from 1 to unbounded

This should close #79.