parse valueless attributes (e.g. itemscope)

And allow nested tags, which was a newly encountered bug
incomplete test for extraxting microdata

See -- TODO
closes #7

Extracting microdata is fairly tedious compared to microformats, due to
its generality. Should probably be included as a standard function of
the parser, or even better, as a separate module, which then can
concern itself with being fully standards compliant.
Merge pull request #11 from wscherphof/issue-#7---microdata

Issue #7   microdata