changed the default encoding of HtmlElement/XhtmlElement.doctype to UTF-8
simplified the definition of HtmElement/XhtmlElement.doctype
add Rakefile to execute unit tests
test_block_head_re_when_commentoutleaf now compares string representations of Regexp instances, instead of the instances themselves.
introduced ELEMENT_TYPE and ELEMENTS_FORMAT to make the assignment to TagFormats clearer
separated the procedure to assign TagFormats into HtmlElement.assign_tagformats
simplified the definition of XhtmlElement class
The intention of the assignment of ELEMENTS_FORMAT in XhtmlElement would become clearer.
just a tidy-up
fixed a bug. XhtmlElement must not share ELEMENTS_FORMAT with HtmlElement.
added a test for empty elements