TODO HTTP::WebTest::Plugin::XMLReport

In no particular order:

o Add plugin parameter "time_zone" for date calculations
  This is needed for DateTest anyway, so why not?

o Enhance output to feed back into WebTest, so we can do
  retries on failure.

o Add sample scripts for post-processing of XML output

o Add XML input module, either as script (proof of concept
  is done) or as an inherited WebTest module.

o Implement missing functionality from DefaultReport:
  - show_headers
  - show_html
  - show_cookies

