Pythonic API design

@nshmyrev this is my idea of "pythonic" api.

```
import vosk

# print available models
print(vosk.list_models())

# auto downloads the model if not found in local
asr = vosk.load("en-us")

# word_list is optional
# if stream=True, will return iterator for partial results
# if stream=False will return final result
# default stream = False
result = asr.recognize('wav_file_path', word_list, stream=True)
```
let me know if this is ok or any changes required.