Adopts sparql's literal value format

[superfell]

The current format for literals is based on sparql, but is not sparql. It uses slightly different format so that the underlying concrete type can be set. (e.g. 10 instead of "10"^^xsd:int). Instead it should take the type from the type specifier, and allow some way for custom schemas to map their types to the xsd types, so you can do "10"^^uom:inch and still get a int64 value.