Simplified the LIMIT condition

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

Exception was possible becayse was by mistake was used a getattr instead
of hasattr

 Exception was possible becayse was by mistake was used a getattr
instead of hasattr

By mistake committed few lines from a test class

removes an unused import

Includes better sparqlestore.triple() documentation

minor text update

CLOSED - task 8: Fetching citation metadata on the server side
https://github.com/cedadev/djcharme/issues/issue/8