Bug fixed.
- Fix DHT11 zero samples bug. 000a52c#diff-f7ea3c8fdb917259dfc681cac4833722R32
- Refine protocol for DHT22. 000a52c#diff-f7ea3c8fdb917259dfc681cac4833722R221
- Write example to stat success rate. #11
Note: Both DHT11 and DHT22 work well under test.