Facet Pivot, histogram widget, and pyramid widget

This pull request contains the changes we have done to integrate the options of facet.pivot, and the two widgets we created. Sadly all our work is based on another database: one with comments published in meneame (a spanish website somewhat similar to reddit), so the data we used may not have any equivalent in the reuter's database.
In any case, the modifications we did in AbstractFacetWidget are all the ones related to pivots, and do not rely on which data is provided by solr.