Free text search: use a custom filter function instead of REGEX.

Using a filter function instead of a magic property (property function)
so we can allow additional arguments later.

Using the namespace <http://d2rq.org/function#> for d2rq-specific
functions, analogous to arq's function namespace
<http://jena.hpl.hp.com/ARQ/function#>.

The textMatches function name is modeled after the langMatches function
name built into sparql.