Allows a graph to limit the number of results from a triple() operation

I realized that in some situation the triples() method may return a huge number of results and if you are implementing something like 'paging' may be a critical performance issue.
Apart from more complicated solution as caching an easy, partial solution is to add a LIMIT, OFFSET and a necessary GROUP BY clauses to the SPARQLStore.triples(). Those parameters can be passed augmenting the Graph object, which is passes as 'context' to the triples method.
I implemented such solution with a few lines and I hope other may agree in this.

For example, using the LIMIT value is as easy as :
```
graph_instance.LIMIT = 10
graph_instance.OFFSET = 2 #retrieve the results from 20 to 30
# GROUP BY is done automatically on the first None of the triple, the subject in this case
graph_instance.triples((None, None, None))
```