Change harvest data format.

Summary:
 - enclose metadata elements in a <metadata> node
 - use path as document id if documentIdXpath is not provided

Test Plan:
bin/docs2harvest -c config/arxiv-harvester.json -o data/corpus-harvests test/data/astro-ph0001197.html
bin/docs2harvest -c config/zbl-harvester.json -o data/corpus-harvests test/data/zbl4138077.xhtml

Reviewers: radu.hambasan

Reviewed By: radu.hambasan

Differential Revision: http://phab.code4fun.de/D77