Network.URI now correctly chosen from network or network-uri

In network-2.6 the Network.URI module was split off from the network
package into the network-uri package. This commit means that rdf4h
automatically gets Network.URI from the right package. See the
documentation at:
http://hackage.haskell.org/package/network-uri

Fixes robstewart57/rdf4h#14
Added network-uri predication on executable and testsuite
cabal version fix to 1.2.7
deprecating `sortTriples` and `removeDupes`; introducing `uniqTriplesOf`
Streamlined quickcheck tests by removing duplication

All tests are now defined in Data.RDF.GraphTestUtils.hs . New graph
implementations only need to provide implementations of

empty'     :: RDF rdf => rdf
mkRdf'     :: RDF rdf => Triples -> Maybe BaseUrl -> PrefixMappings -> rdf
triplesOf' :: RDF rdf => rdf -> Triples

See Data.RDF.MGraph_Test.hs for an example. Add your graph
implementation to `defaultMain` in Test.hs .