Free text search for postgresql, a hack.

This is a first attempt at implementing a free text search mechanism.
It is wrong in a number of ways.

* The REGEX filter is abused to represent the free text query.
  We should use a triple pattern, where the property is a
  built-in function recognized by D2RQ.

* SQL for free text search should depend on the db vendor.

* At the moment, this bombs when trying to search on a non-text column.
  We should inspect the datatype of the attribute before proceeding.