Allow additional recognized URIs in model

The importer on Joinup does not recognize ADMS-AP yet and chokes when
receiving more properties than it understands. For now, just recognize
those ADMS-AP URIs when parsing a graph, but do not generate them.
Separate validator from graph creation

This patch makes the validator implementation much cleaner.
Update model constraints
Add Repository to model

Processors must now return a Repository object instead of a graph.
Validation is done at upper level for all repositories.
More validation rules
Do not remove triples when extracting model

If you need the residual graph, convert the model into a graph and
compute the difference.
w3c: Quick fixes
Prettify log
w3c: Use Last-Modified HTTP header