added 'to_pattern()' utility for pattern expressions; more information on patterning guide

stix2/patterns.py

@@ -40,7 +40,7 @@ class TimestampConstant(_Constant):
         try:
             self.value = parse_into_datetime(value)
         except Exception:
-            raise ValueError("must be a datetime object or timestamp string.")
+            raise ValueError("Must be a datetime object or timestamp string.")
 
     def __str__(self):
         return "t%s" % repr(self.value)