HTML Tags converted to lower case and xhtml doctype

Hi,

first thanks for all the effort put into question to answer. With this pull request I want to suggest a small series of changes.

In preparation for a custom template I converted the generated HTML of question2answer to lower case HTML and changed the doctype to xhtml 1.0 transitional.

Additionally I fixed smaller html errors like missing closing tags.


With these changes valid xhtml or html5 templates should be easier to realize. If you have comments to this patch or would suggest some modifications please let me know.