closes #6

:not(), [att=val], [att!=val], [att|=val], [att*=val], [att~=val],
[att^=val], [att$=val]
Note that the selection is now returned as a simple Set, breaking the
abilty brought in by #8 and #9 tot :select() or () on the selection. Of
course, the elements in the returned Set are still ElementNodes that
can be selected upon.