Provide an optional threaded background worker

Due to the three background workers (scheduler, twitter stream and delayed job)
huginn needs a lot of memory to run (about 520MB on my dev machine). This PR
introduces an optional threaded background worker which combines the three current
separated processed into one (reducing the memory footprint to ~260MB).
Since just one instance of the of every background processor is running at a time
there should not be any threading related issues.
The main gotcha of this is, that it's most likely not possible to run multiple
delayed job workers concurrently. The ultimate solution would probably be switching
to sidekiq with sidetiq as scheduler, but that is a different task :)

When running on MRI the GIL should not be an issue because it is released for most IO
bound operations (waiting for the database/website/sleeping).
Use threaded workers in the chef production Procfile
Add SlackAgent

The SlackAgent lets you receive events and send notifications to slack
Add default username
Updated Cheffile.lock to work with ubuntu 14
Make agent name link to show
Fix menu/actions drop down regressions from previous patch (+ add bootstrap style colour classes to css)
Handle agent disabled opacity a bit nicer
Add 'working' link for disabled agents
Extract actions menu into partial and use it for both Agent Index and Show pages
Shuffle the edit menu item down one
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