Declare a dependancy on pydot

Not sure this is the right place

setup.py

@@ -50,6 +50,9 @@ setup(name=name,
       install_requires=[
         'setuptools', # namespaces
         'psutil >= 0.5.0',
+        # XXX for erp5_graph_editor prototype
+        'pyparsing == 1.5.7',
+        'pydot',
       ],
       extras_require={
         'testnode': ['slapos.core', 'xml_marshaller', 'psutil >= 0.5.0'],