symmetrically encrypted RSS platform

[hupili]

A RSS2RWEnc class derived from RSS2RW.

• add func: RSS2RW._update_to_string.
• Reroute RSS2RW._update to RSS2RW._update_to_string
• RSS2RWEnc.update use RSS2RW._update_to_string and encrypt before output to files.
  • Intercept RSS.home_timeline. feedparser._open_resource is a good way to get all kinds of resources, e.g. URL, local file, etc. Decrypt before passing the stream to feedparser.

The snscrypt.py was there for one year but not used. Let's revive it. Some upgrades before using it:

• Use the utils.obj2str and utils.str2obj for serialization. Current base64 can not handle unicode.