Integration of a worker queue backend

[seebi]

we need a backend abstraction for dropping worker tasks such as:

• do a semantic ping
• push new content
• email contribtutors
• create all resources for the site extension
• re-scale images

some tools for doing this are: gearman, beanstalkd, apache activeMQ and more

as a fallback, we need cron and instant worker queues.

• cron: save tasks somewhere, where a simle run script can start it
• instant: do not save tasks, just do them instantly (fallback if nothing is configured)

[seebi]

maybe interesting list:

• http://squirrelshaterobots.com/programming/php/building-a-queue-server-in-php-part-1-understanding-the-project/

[seebi]

please start a new feature branch for that issue
hope to hear soon from you here