Cleanup in Wiki text section


* Drop the ParsedPage class. It did not provide extra functionality and is easy confused with ParsingPage.
* Move page rendering logic (wikitext style) to BaseWikiPage
* Make VariableSource a property in FitNesseContext, since it's used on several locations. Instantiating SystemVariableSource all over the place is also confusing.
* Cleaned up TestPage interface (WikiTestPage is also rendering).