Check for empty reference snak list/dict in JsonReader

File "[...]/propertysuggester/parser/JsonReader.py", line 47, in _process_json
    for ref_id, snaks in reference["snaks"].iteritems():
AttributeError: 'list' object has no attribute 'iteritems'
Merge pull request #14 from Wikidata-lib/fixNoIteritems

Check for empty reference snak list/dict in JsonReader