Extended LinqTests to test the DotNetRdfDataObjectContext class
Initial implementation of use of CONSTRUCT
Fixed CONSTRUCT query to use a subquery

This ensures all triples are returned for matching entities.
XDocument now kept as private variable

Ensures the ResultDocument property can be accessed multiple times when
it comes from a stream.
Test updates
Linq and SPARQL updates

Fixed LINQ query issues against generic SPARQL endpoints
Changed the SPARQL query builder to create a CONSTRUCT rather than a
DESCRIBE query to retrieve entities in full.