Added ability to set custom lock key

Like in [defunkt/resque-lock](https://github.com/defunkt/resque-lock):

```ruby
class UniqueJob
  include Sidekiq::Worker

  sidekiq_options :queue => :unique_job, :unique => true

  def self.lock(id)
    'sidekiq:lock:unique_job'
  end

  def perform(id)
    # ...
  end
end
```