Add combined position_path field.

The search dashboard stores a document for each combination of (search, document path, document rank), which holds a count of the number of times the document was clicked on.

The dashboard then performs a terms_stats facet (http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-facets-terms-stats-facet.html) to calculate the sum of the click counts for each document or for each position for a given search.  However, we can't calculate the sum of the click counts for the combination of each position and document, which is what I actually want to use.

This allows us to can calculate counts of number of times a result is
clicked on in a given position for a search.  Wouldn't be needed if we
were using elasticsearch 1.0+ aggregations which allow a script to be
used to calculate the key for this sort of operation.