Change mws:harvest format to version 2.

Summary:
Add <mws:data> and mws:data_id to link between expressions and data elements.
This is backwards compatible with harvest version 1.
See harvest specification at [1].

[1] https://trac.mathweb.org/MWS/wiki/MwsHarvest

Test Plan:
Run on arxivdemo dataset [2].

[2] https://svn.mathweb.org/repos/mws/trunk/core/data/arxivdemo/

Reviewers: radu.hambasan, dhasegan

Reviewed By: radu.hambasan

Differential Revision: http://phab.code4fun.de/D32