(WIP) Sidekiq integration

Hello gang :) We at @flockler added a Sidekiq-Opbeat integration into our app and figured we'd throw it to you if you would like it into the gem.

**Note** This is still a work-in-progress! **Do not merge yet!** For some reason, the `Sidekiq.configure_server` doesn't seem to do anything when I cargo-culted the code over to this gem. Works perfectly in our Rails app, so it's probably some kind of dependency or configuration issue. Help is greatly appreciated!

I'll let you know if I get it fixed :)
I explicitly defined Sidekiq 2.x because adding an error handler to 2.x is much more complicated than Sidekiq 3.x.

P.S. Any idea how to do tests for two different versions? ;)