New filter: elapsed

I wrote a new filter called  "elapsed".

The elapsed filter tracks a pair of start/end events and uses their timestamps to calculate the elapsed time between them: 

The configuration looks like this:

     filter {
       elapsed {
         start_tag => "start event tag"
         end_tag => "end event tag"
         unique_id_field => "id field name"
         timeout => seconds
         new_event_on_match => true/false
       }
     }

Inside the filter file I wrote a longer and more detailed description. I wrote also a spec file.