Store html content in file when the xml is parsed

The HTML contents was saved in memory and then stored in files once the whole result file has been parsed. 
To avoid OOM, I write the files when the XML is parsed.

I think we could close JENKINS-15863 & JENKINS-13936.

Note : during my tests, I detect that the "Detail" link is not working in case of execution fitnesse test which is not a suite (I think, it is linked to pull request 10).