Add SlackAgent

The SlackAgent lets you receive events and send notifications to slack
Add default username
Updated Cheffile.lock to work with ubuntu 14
check for nil in ensure part
Merge pull request #347 from peter-x/patch-1

check for nil in ensure part
Merge pull request #343 from alias1/disable-agents-tweaks

Disable agents tweaks
Merge pull request #345 from dsander/update-chef-recipes

Updated Cheffile.lock to work with ubuntu 14
fix "working state" navigation to logs; remove underlining on "working state" links
Merge branch 'master' of github.com:cantino/huginn
Added a liquid tag to access user credentials

Via '{% credential USER_CREDENTIAL_NAME %}' liquid enabled agent options
can be used to access stored user credentials
Merge pull request #348 from dsander/liquid-credentials

Added a liquid tag to access user credentials
Merge branch 'master' into threaded-background-workers
Threaded background worker now is the default
Merge pull request #342 from tonmoy-saikia/slack_agent

Add SlackAgent
Solves #354
Merge pull request #318 from dsander/threaded-background-workers

Provide a new threaded background worker
Add simple MQTT support to subscribe to, publish messages.
Tweak wording
Assume the payload is going to be JSON
Indent description