Replace Clockwork with Sidetiq

"We should eliminate clockwork, its just wasting memory for no reason, https://github.com/tobiassvn/sidetiq8 seems like a reasonable replacement that can run inside sidekiq." 
http://meta.discourse.org/t/unbound-memory-usage/8137/6

Here you go!

![schermafbeelding 2013-08-07 om 18 53 25](https://f.cloud.github.com/assets/6944/926219/92de8ff2-ff87-11e2-9b6b-8d2dde38a3bc.png)