Add dict support

[sergseven]

Expected tests to pass:

jason_encode_dict_test() ->

  ?assertEqual("{}", jason:encode(dict:from_list([]))),
  ?assertEqual("{\"abc\": \"def\"}", jason:encode(dict:from_list([{<<"abc">>, <<"def">>}]))),
  ?assertEqual("{\"abc\": \"def\",\"abc2\": \"def2\"}", jason:encode(dict:from_list([{<<"abc">>, <<"def">>}, {<<"abc2">>, <<"def2">>}]))),

  ?assertEqual(
    "{\"abc\": {\"k1\": 1,\"k2\": \"ab\"},\"abc2\": {\"k1\": 2,\"k2\": \"cd\"}}",
    jason:encode(
      dict:from_list([
        {<<"abc">>, {r, 1, <<"ab">>}},
        {<<"abc2">>, {r, 2, <<"cd">>}}]),
      [{records, [{r, [k1, k2]}]}])),

  ok.