new getter functions: rasqal_graph_pattern_get_triples, rasqal_literal_get_type, rasqal_literal_get_language

I've created 3 new simplistic getters for accessing private members of the rasqal_literal and rasqal_graph_pattern structs.  The rasqal_graph_pattern_get_triples creates a new raptor_sequence containing only the triples of the graph pattern, or NULL if no triple patterns are contained in the respective graph pattern. The functions pertinent to the rasqal_literal struct are exposing the type and language members with minimal checks.