Fixing tests.

- Prefer urllib fetcher to pycurl, as the latter refuses to work
  (and we definitely don't want to fix that, too)
- Don't decode received payload into a string, as an xml parser
  wants bytes.
- Fix str/bytes discrepancies everywhere.
Removed CurlFetcher and Httplib2Fetcher.
Removed base class HTTPFetcher in favor of duck typing.
Removed unused imports in fetchers.
Replaced custom HTTPFetchingError with urllib.error.URLError