Add complex harvests to xhtml2harvests.

Summary:
Add extended harvest support in crawler.

Harvest now supports in each expression a <content> node and a
<data> node. The content node is similar to the old version (Content
MathML), while the <data> node cand be any valid XML. This will be
stored by MWS and returned with results.

Crawler now indexes the entire MathML node, to allow snippets of
cross-referenced content and presentation MathML.

Test Plan:
Run
> ./xhtml2harvests ../test/data/zbl4138077.xhtml
Check harvest_*.harvest created.

Reviewers: m.iancu

Reviewed By: m.iancu

Differential Revision: http://phab.code4fun.de/D12