Session support and HTML reporting added

A much-requested feature: it's now possible to have <httpRequest> tests share a common session, via the <session> nested element. See 'test.xml' for an example of how this is done.

Also, XML logging and HTML reporting is now implemented. The XML logs are in the same format as those from the <junit> task, so we can reuse the <junitreport> task to create some nice-looking HTML reports. An example of a generated report is (temporarily) up at http://aft.sf.net/report/frames/

--Jeff