Improvedment as discussed in issue 9

## Speed improvments
about 20%
```
test/bench_tests.erl:33:<0.119.0>:   serialize: 0.479 s
test/bench_tests.erl:34:<0.119.0>: deserialize: 0.684 s
test/bench_tests.erl:35:<0.119.0>: for 2011 KB test data(msgpack_jiffy).
test/bench_tests.erl:39:<0.119.0>:   serialize: 0.474 s
test/bench_tests.erl:40:<0.119.0>: deserialize: 0.695 s
test/bench_tests.erl:41:<0.119.0>: for 2011 KB test data(msgpack_jsx).
test/bench_tests.erl:45:<0.119.0>:   serialize: 0.028 s
test/bench_tests.erl:46:<0.119.0>: deserialize: 0.073 s
test/bench_tests.erl:47:<0.119.0>: for 1884 KB test data(msgpack_nif).
test/bench_tests.erl:51:<0.119.0>:   serialize: 0.073 s
test/bench_tests.erl:52:<0.119.0>: deserialize: 0.056 s
test/bench_tests.erl:53:<0.119.0>: for 3828 KB test data(t2b/b2t).
```
## JSX and jiffy support
A second argument can be passed to pack/unpack with either `[jiffy]` or `[jsx]`, it defaults to `[jiffy]` so it does not break current code.