1.2.0 Version

This release feature a few breaking changes on some api calls.

Here's an overview of the main features embedded in this release:

New features:

Bug fixes:
#158 : a listener can now safely add or remove some listener (including itself)
#180 : There should not be anymore multiple websocket connections at the same time
#184 : The bot shouldn't receive multiple time the same event (was caused by #180)
#215 : no more NPE when passing null as a channel in com.ullink.slack.simpleslackapi.SlackSession#sendMessage

new features :
JSON content of unsupported messages is now logged (instead of logging UNKNOWN event)
#165: It is now possible to retrieve a specific message from a channel using its timestamp
It is now possible to retrieve all kind of message in the channel history
sending ephemeral messages is now possible
message attachment update is now possible

Many thanks to the contributors.