[IMP] convert qweb to use lxml instead of minidom

should be >20x faster at parsing document strings and gain close to 40s on crawling
[FIX] don't call render_node on comments (or PIs)
Added required python-pypdf package

It's needed because fc8592adf2f26bf35007d6cc625fb2005cd65b5d is now loading the `pyPdf` package.
Merge pull request #881 from xmo-odoo/master-qweb-lxml-xmo

Convert qweb (Python) to use lxml
Merge pull request #884 from rewritten/patch-1

[FIX] debian/control: add required python-pypdf package