Added support for "not_in" operator when using text fields (#193)

Thank you for your contribution @hasielhassan !

Author: hasielhassan

shotgun_api3/lib/mockgun/mockgun.py

@@ -607,6 +607,8 @@ def _compare(self, field_type, lval, operator, rval):
                 return lval.startswith(rval)
             elif operator == "ends_with":
                 return lval.endswith(rval)
+            elif operator == "not_in":
+                return lval not in rval
         elif field_type == "entity":
             if operator == "is":
                 # If one of the two is None, ensure both are.