Adding functionality to return a scheduled job given a job id

Had a similar issue as @jmazzi (https://github.com/mperham/sidekiq/issues/536#issuecomment-10604878) where I needed information on a scheduled Sidekiq job.   

Example usage:

Sidekiq::Client.get_scheduled_job('b0e879cbe7fcf3460acc2b67')
 => {"retry"=>true, "queue"=>"default", "class"=>"Sidekiq::Extensions::DelayedClass", "args"=>["---\n- !ruby/class 'MyClass'\n- :test\n- []\n"], "at"=>1366142912.6503942, "jid"=>"b0e879cbe7fcf3460acc2b67"} 

Please let me know if I've totally overlooked a better way to do this.

Cheers!