Feature 477 - Support Cloud Files public storage

*   Added configuration options to mediagoblin.ini
*   process_media supports the python-cloudfiles
    almost-file-like objects by wrapping them in a
    contextlib.contextmanager-decorated func.
*   storage now has the CloudFilesStorage
*   New dependency added to setup.py; `python-cloudfiles`

setup.py

@@ -44,6 +44,7 @@ setup(
         'webtest',
         'ConfigObj',
         'Markdown',
+        'python-cloudfiles',
         ## For now we're expecting that users will install this from
         ## their package managers.
         # 'lxml',