Add an extras_require dependency for cElementTree if we want to use fast iterparsing.

setup.py

@@ -78,6 +78,7 @@ dist = setup(
                           'COPYRIGHT.txt'])],
     install_requires = ['medusa >= 0.5.4', 'meld3 >= 0.6.3',
                         'elementtree >= 1.2.6'],
+    extras_require = {'iterparse':['cElementTree >= 1.0.2']},
     include_package_data = True,
     zip_safe = False,
     namespace_packages = ['supervisor'],