feed: Implement feed.fetch()

feed: Implement feed.fetch()

Add __version__, __url__, and USER_AGENT to the package namespace, so
we have a useful user agent to use when fetching.  The fetch code can
hard-code UTF-8 as the feed charset, because RFC 5545, section 6
(Internationalization Considerations) reads:

  Applications MUST generate iCalendar streams in the UTF-8 charset
  and MUST accept an iCalendar stream in the UTF-8 or US-ASCII
  charset.

From a character conversion standpoint, that means we can always use
UTF-8, because US-ASCII is a subset of UTF-8.