Flexible device type

@dwbutler
- I added a spec for the previous PR https://github.com/dwbutler/logstash-logger/pull/18
- I updated `device_klass_for` to accept more flexible input.

I using environment variables to set the device type to be different in staging vs production.

```
ENV['LOGSTASH_URL'] = "tcp://localhost:5229"

# ...

logstash_url = URI.parse(ENV['LOGSTASH_URL'])

config = {
  type: logstash_url.scheme.to_sym,
  host: logstash_url.host,
  port: logstash_url.port
}
``` 

As you can see above `logstash_url.scheme` returns a `String` vs a `Symbol`

This PR will allow users to not have to explicitly convert the type to a `Symbol`

-- Arron