Adding datetil to macros and documenting macros references

airflow/macros/__init__.py

@@ -1,6 +1,7 @@
 from __future__ import absolute_import
 from random import random
 from datetime import datetime, timedelta
+import dateutil
 import time
 from . import hive
 import uuid

docs/code.rst

@@ -138,7 +138,26 @@ attributes and methods.
 
 Macros
 ''''''
-These macros live under the ``macros`` namespace in your templates.
+Macros are a way to expose objects to your templates and live under the 
+``macros`` namespace in your templates.
+
+A few commonly used libraries and methods are made available.
+
+=================================   ====================================
+Variable                            Description
+=================================   ====================================
+``macros.datetime``                 The standard lib's 
+                                    ``datetime.datetime``
+``macros.timedelta``                 The standard lib's 
+                                    ``datetime.timedelta``
+``macros.dateutil``                 A reference to the ``dateutil``
+                                    package
+``macros.time``                     The standard lib's ``time``
+``macros.uuid``                     The standard lib's ``uuid``
+``macros.random``                   The standard lib's ``random``
+=================================   ====================================
+
+Some airflow specific macros are also defined:
 
 .. automodule:: airflow.macros
     :show-inheritance: