Enhance PropertyMapping with element selection

This add the possibility to select either the first or the last element in a list
of template property values, in order to map to an ontology property
Merge pull request #31 from ninniuz/MapPropertySelector

Map property selector
comment: stuff like \u042F is JSON code, not HTML

see http://tools.ietf.org/html/rfc4627#section-2.5
Merge branch 'master' of https://github.com/dbpedia/extraction-framework