Added a new filter::redis

This is a new filter which is able to read/write from a redis server.

It was very helpful for me when trying to get some piece of information from one line of log, and put it back into another one, later on.

YYYYMMDD-HHMMSS LOG-line ...... Information A is xxxxx <--- use filter {grok and redis} to STORE 'xxxxx'
YYYYMMDD-HHMMSS LOG-line ...... 
YYYYMMDD-HHMMSS LOG-line ...... 
YYYYMMDD-HHMMSS LOG-line ...... Some line B <--- use filter{grok and redis} to READ 'xxxxx' and store it in field A

This gives : 

{
   "message" => "YYYYMMDD-HHMMSS LOG-line ...... Some line B"
   "A" => "xxxxx"
}