First pass as Plantain, an immutable RDF implementation in Scala.

URIs are from Spray and the Turtle parser/writer are from Sesame.
refactoring in PlantainTurtleWriter
implemented graph diff in Plantain
Implemented Graph isomorphism for Plantain using Sesame's
small refactoring in TurtleTestSuite to rely on typeclasses
wired test for Turtle parser/writer in Plantain
wired all tests that could be wired in Plantain
All methods in URISyntax are directly derived from URIOps.

URIOps is now required for RDFOps and a DefaultURIOps is made available.
Provided a custom PlantainURIOps.

PlantainURIOps.relativize() still relies on java.net.URI because
spray.http.Uri does not expose a similar method.
PlantainURIOps.getString should defer to the underlying URI
Added reference to https://github.com/spray/spray/issues/818
PlantainOps.graphToIterable just delegates to Graph.triples
moved PlantainURIOps in its own file
[plantain] fix the isPureFragment implementation
[plantain] more tests and fixes related to PlantainURIOps
[plantain] added some comments about RDF Graph isomorphisms
Merge pull request #102 from w3c/plantain

some things to be done still and to be taken into consideration ( especially named blank nodes), but these can be done in further commits