'Get a specific block for a given public key
:param public_key: The public_key for which the block has to be found.
:param sequence_number: The specific block to get
:return: the block or None if it is not known'

def get(self, public_key, sequence_number):

    return self._get(u'WHERE    public_key    =    ?    AND    sequence_number    =    ?', (buffer(public_key), sequence_number))
