'unused' prefix to suppress warnings of unused variables

In addition to '_', also ignore any unused variables whose name start
with 'unused', as recommended by http://google-styleguide.googlecode.com/svn/trunk/pyguide.html?showone=Lint#Lint

Cython/Compiler/FlowControl.py

@@ -634,7 +634,7 @@ def check_definitions(flow, compiler_directives):
     for entry in flow.entries:
         if (not entry.cf_references
                 and not entry.is_pyclass_attr):
-            if not entry.name.startswith('_'):
+            if entry.name != '_' and not entry.name.startswith('unused'):
                 # '_' is often used for unused variables, e.g. in loops
                 if entry.is_arg:
                     if warn_unused_arg: