Add "age" based boosting to ezfind/solr relevancy score

the current "relevancy" sorting in solr does not take into account the date of the documents.

The desired solution is to have a proper age-based boost factor to tune the relevancy score of matched documents.
an introduction to the problem can be found here:
http://wiki.apache.org/solr/SolrRelevancyFAQ#How_can_I_boost_the_score_of_newer_documents