store:
- Use utils.get_type_from_id() instead of my own (I didn't know it
was already there)
- Use dict-style instead of attribute-style access to get stix
object properties
- Convert timezone-aware timestamps to UTC in _timestamp2filename()
to ensure that different times always result in different
filenames.
Also added a couple new tests for _timestamp2filename(), which
exercises the timezone conversion code.
Factored out the _is_marking() function from the memory datastore
module to utils so it can be reused, and changed both filesystem
and memory datastore modules to import and use it.
Michael Chisholm