Integrate DOSS

First crack at integrating DOSS into AmberDb (#12). This is very rough but works and I didn't want us going too far down the "FileLocation" path.

Reworks the API to favour the Java 7 Path class rather than old java.io.File. That has the nice side-effect of avoiding the java.io.File vs amberdb.model.File confusion.

I've had to wrap the graph (see AmberDb.OwnedGraph) in order to store a back-reference to the AmberDb object to get a handle on the BlobStore. An alternative would be to add it to AmberGraph which I decided against for now as it breaks layering and means we can't test with TinkerGraph which matters until AmberGraph is turned on. Another option would be to subclass FramedGraph but that looked more annoying as you have to replace the factory and configuration stuff too.