Allow retrieval of index entries

This is still very basic, as we only actually load three fields into
the record.

c_src/geef.c

@@ -179,6 +179,9 @@ static ErlNifFunc geef_funcs[] =
 	{"index_write_tree", 1, geef_index_write_tree},
 	{"index_write_tree", 2, geef_index_write_tree},
 	{"index_add",        2, geef_index_add},
+	{"index_count",      1, geef_index_count},
+	{"index_get",        3, geef_index_get},
+	{"index_nth",        2, geef_index_nth},
 	{"index_clear",      1, geef_index_clear},
 	{"index_read_tree",  2, geef_index_read_tree},
 	{"signature_new", 2, geef_signature_new},