support python 2.5.4+ for marshaller/demarshallers

Patch adds a "from __future__" import that doesn't affect newer python's but
allows python 2.5.4 to run the code (tested under scratchbox, n900 build environment)

python_modules/codegen.py

@@ -1,3 +1,4 @@
+from __future__ import with_statement
 from cStringIO import StringIO
 
 def camel_to_underscores(s, upper = False):

python_modules/demarshal.py

@@ -1,3 +1,4 @@
+from __future__ import with_statement
 import ptypes
 import codegen
 

python_modules/marshal.py

@@ -1,3 +1,4 @@
+from __future__ import with_statement
 import ptypes
 import codegen