adding html5lib, since html.parser does not normalise HTML well

Specifically, given the input

"<p>foo<br>bar</p>"

html.parser normalises this by closing the br tag:

"<p>foo<br>bar</br></p>"

Firefox then treats this as:

"<p>foo<br></br>bar<br></br></p>"

Note that beautifulsoup uses html5lib automatically if it is
installed.
fixing assertEqualIgnoringWhitespace to actually ignore whitespace correctly