Ability to track progress, fix for #3, major refactoring

I noticed that Sidekiq already store job id in Worker class (see https://github.com/mperham/sidekiq/blob/master/lib/sidekiq/worker.rb#L24) and so refactored Server middleware in the way so it's no more dependent on Status::Worker, which allowed me to significantly simplify the code and it also fixes this bug (https://github.com/utgarda/sidekiq-status/issues/3), other changes are rather trivial. Here is what was made:

* Refactored out dependency on Status::Worker of ServerMiddleware;
* Added ability to track progress in the same way as `resque-status` does with `at` function;
* Updated README to the all changes I made and to the changes introduced in #4.

All tests are green of course.