[#6458] Add google-code wiki page extraction

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
[#6458] Refactored Google Code project extractor

- Introduced a caching get_page() method so that page fetching
  is no longer tightly coupled to extractor instantiation. In
  other words, you can fetch any page you want, regardless of
  the page that you instantiated the extractor with.

- Also removed the wiki-specific stuff and moved that to a
  subclass in the googlecodewikiimporter pkg.

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>