Improved multiline comment parsing.

Fixes #257 and slightly alters comment parsing behavior.

Unlike self-closing tags, a comment can contain angle brackets between the
opening and closing tags. The greaterthan angle bracket at the end of the
first block should not be mistaken for closing the comment. Need to actually
check for a comment closing tag (`-->`). If one if not found, then the comment
keeps going (to the end of the document if nessecary) just like in HTML.

That last bit is a slight change from previous behavior, but should be
unsurprising as that's how broswers parse html comments. And as far as
I can tell, more implementations follow this behavior than any other. The
ones that don't seem to be all over the place.
Merge branch 'master' of github.com:waylan/Python-Markdown