[redland] Statement groups in Redland

[Dave Beckett]

This is to note some progress - I've added two new methods to the
model and storage system so that you can 1) add sets of statements to
a model in a URI-labelled group and 2) retract all of them.

This means that for example, you can parse a load of RDF/XML files
into a redland store, retract the files in any order and get what you
expect.  Uses of this could include tracking provenance, talking
about collections of statements (for example N3 contexts) and
handling updating aggregated content.

This is currently just implemented for the in-memory storage system
while I get the test and corner cases working (done).  The next step
is to do it for the persistent storage - Berkeley/Sleepycat DB -
which should be reasonably straightforward.

You can get this stuff in the nightly snapshots below 
  http://www.redland.opensource.ac.uk/dist/snapshots/

Dave