sparql store now supports limit clause, closes #346

Allows a graph to limit the number of results from a triple() operation
adding a SPARQL 'LIMIT' clause.
For instance, the LIMIT value may be set adding
graph_instance.LIMIT = 10

Adds OFFSET and GROUP BY to the triples() method in order to allow
pagination and reduce the number of results

Signed-off-by: Joern Hees <dev@joernhees.de> [squashed + whitespace]