Redis device

This adds a redis device that can be used for sending logs to a redis server. It can push logs to a list or channel to make it fully compatible with the logstash redis input.

```ruby
  device = LogStasher::Device::Redis.new(:host => 'redis1.example.com', :key => 'logstash')
  LogStasher.logger = Logger.new(device)
```

Not sure if this is within the scope of your project, but thought I'd let you take a look anyway.