Replaced createHTTPFetcher call with its only return value.
Inlining _lowerCaseKeys as a dict comprehension.
Removed unused parseHeaderValue. (And this should have been cgi.parse_header in the first place.)
Moving _makeResponse out of the class as it doesn't access any attributes.
Removing static alias "urlopen". Now Urllib2Fetcher is equivalent to a function.
Removed last fetcher class.

- Urllib2Fetcher becomes a module-level function "fetch".
- Pseudo-singleton _default_fetcher removed
- Tests manipulate fetchers.fetch directly (for now?)
Moved testing disallowed urls into test_fetcher.
Inlined _allowedURL
Removed unnecessary delegation through run_fetcher_test