Remove the isstring hack in lxml treewalker.
Remove the isstring hack in lxml treewalker.

This fixes #115 as a result, as we're no longer type-sniffing strings.

mindw/html5lib-python,mgilson/html5lib-python,mgilson/html5lib-python,dstufft/html5lib-python,html5lib/html5lib-python,ordbogen/html5lib-python,html5lib/html5lib-python,dstufft/html5lib-python,alex/html5lib-python,dstufft/html5lib-python,mindw/html5lib-python,mindw/html5lib-python,alex/html5lib-python,ordbogen/html5lib-python,ordbogen/html5lib-python,alex/html5lib-python,html5lib/html5lib-python,mgilson/html5lib-python