Elixir Slack Versions Save

Adds the unfurl action from the Slack Web API

Patch fix for default state bug

Downgrade ex_doc to ~> 0.18 to allow hex.publish on Elixir 1.6

Adds missing Slack Web API methods:

• admin.apps.*
• app.uninstall
• auth.revoke
• chat.scheduleMessage
• oauth.v2.access
• users.profile.get
• users.profile.set
• views.*

Fixes messaging warnings

superfluous_charset warnings thrown with every im.open call are now properly handled with a default Content-Type of utf-8 (it was missing).