add explicit classes that denote tag-level changes

[anastasia]

d = HTMLDiffer('<h1>whatever</h1>', '<div>whatever</div>')
d.diff()'s combined_diff should potentially look like
'<div class="htmldiffer-tag-change">whatever</div>'
and attribute-level changes could look like:
d = HTMLDiffer('<div class="something">whatever</h1>', '<div class="something_else">whatever</div>')
d.diff()'s combined_diff should potentially look like
'<div class="htmldiffer-attribute-change">whatever</div>'
or
'<div class="htmldiffer-class-change">whatever</div>'

On top of current wip enhancement, would deal with #27, #28, #29