Streaming RDF export

[armisael]

With these changes the RDF export becomes streaming:

• the data is not handled twice;
• the triples are not loaded in memory as a whole, but row by row;
• the output generation starts while reading the rows.

Fixes issue #101

[armisael]

My IDE messed up imports and indentation, I'll try to:

• amend imports and indentation (I fear it will be painful)
• deprecate the buildModel method
• throw RuntimeExceptions instead of just logging

Thanks