minor fixes:

* export Message in the top level
* dont try to decode old IDENTIFY 'OK' response (this wasn't *broken*
  but this clears up a misleading log message)
Merge pull request #23 from mreiferson/fixes_23

export Message and minor fix
fix JSONDecodeError, it is only existed in simplejson
add is_starved and update comments
Merge pull request #24 from mreiferson/starved_24

add is_starved method
Add async Writer to support pub&mpub message
Merge pull request #25 from felinx/master

Add async Writer to support pub&mpub
fix changelog formatting
bump to 0.4.2; update ChangeLog