Fix/core save api (#172)

* Acknowledge messaages from Pulsar, doh!
* Change API to deliver a boolean e if value is an entity
* Change loaders to use new API
* Changes, entity-aware API is complete

trustgraph-base/trustgraph/knowledge/defs.py

@@ -23,3 +23,11 @@ URL = 'https://schema.org/url'
 IDENTIFIER = 'https://schema.org/identifier'
 KEYWORD = 'https://schema.org/keywords'
 
+class Uri(str):
+    def is_uri(self): return True
+    def is_literal(self): return False
+
+class Literal(str):
+    def is_uri(self): return False
+    def is_literal(self): return True
+