some more documentation updates and minor code cleanups.  Additionally True and true in the template are the same now, same for false/False and none/None.

--HG--
branch : trunk
diff --git a/jinja2/__init__.py b/jinja2/__init__.py
index cd720a6..194390a 100644
--- a/jinja2/__init__.py
+++ b/jinja2/__init__.py
@@ -50,7 +50,7 @@
 # decorators and public utilities
 from jinja2.filters import environmentfilter, contextfilter
 from jinja2.utils import Markup, escape, clear_caches, \
-     environmentfunction, contextfunction
+     environmentfunction, contextfunction, is_undefined
 
 __all__ = [
     'Environment', 'Template', 'BaseLoader', 'FileSystemLoader',
@@ -59,5 +59,5 @@
     'TemplateError', 'UndefinedError', 'TemplateNotFound',
     'TemplateSyntaxError', 'TemplateAssertionError', 'environmentfilter',
     'contextfilter', 'Markup', 'escape', 'environmentfunction',
-    'contextfunction', 'clear_caches'
+    'contextfunction', 'clear_caches', 'is_undefined'
 ]