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).