Add PDF metadata parsed from HTML. Fix #77.

<title> → /Title
<meta name=author> → /Author
<meta name=description> → /Subject
<meta name=keywords> → /Keywords
<meta name=generator> → /Creator
<meta name=dcterms.created> → /CreationDate
<meta name=dcterms.modified> → /ModDate
"WeasyPrint vX.Y" → /Producer