converted unit tests, started rewriting filters

--HG--
branch : trunk
diff --git a/jinja2/nodes.py b/jinja2/nodes.py
index ecc3f1e..02eb3c0 100644
--- a/jinja2/nodes.py
+++ b/jinja2/nodes.py
@@ -405,7 +405,7 @@
             raise Impossible()
         filter = self.environment.filters.get(self.name)
         if filter is None or getattr(filter, 'contextfilter', False):
-            raise nodes.Impossible()
+            raise Impossible()
         if obj is None:
             obj = self.node.as_const()
         args = [x.as_const() for x in self.args]
@@ -423,7 +423,7 @@
         try:
             return filter(obj, *args, **kwargs)
         except:
-            raise nodes.Impossible()
+            raise Impossible()
 
 
 class Test(Expr):
@@ -452,7 +452,7 @@
         try:
             return obj(*args, **kwargs)
         except:
-            raise nodes.Impossible()
+            raise Impossible()
 
 
 class Subscript(Expr):