Add new API chewing_set_logger
Add unit test for logger
Add log level
Add document for chewing_set_logger
Update version due to chewing_set_logger API
Merge branch 'logger'

* logger:
  Update version due to chewing_set_logger API
  Add document for chewing_set_logger
  Add log level
  Add unit test for logger
  Add new API chewing_set_logger