Restructure Formatters (reporting) code.

Clean up the code in the fitnesse.reporting package.

 - For time stamps in page history, the time stamps registered by the
   test history formatter is used to link to the test history page.
 - Removed CachingSuiteXmlFormatter. Now a "reformatter" is used that's using
   the SuiteHistoryFormatter as input.
 - Make attributes private
 - Removed PageHistoryFormatter, it has the same output as XmlFormatter
   (now TestXmlFormatter)
 - Remove responsibilities from BaseFormatter.