DropwizardGelf doesn't work properly

Hi,

Athough I've made the update for Dropwizard 0.7.0 I haven't tested it properly and now that we've upgraded few projects, it seems it doesn't actually work. Dropwizard 0.7 introduced a new way of defining custom loggers, so the ConfiguredBundle is not necessary anymore. Here is a set of changes for that. It makes the library much smaller.

Can you please have a look, if you agree with these changes? I've tested them against Greylog server and it does work. It also needs documentation changes, but I first thought I would check if you agree with these changes in first place.

The logging configuration now looks like this:
```
server:
  requestLog:
    appenders:
      - type: gelf
        facility: FACILITY
        host: HOST
        includeFullMDC: true
logging:
  level: INFO
  appenders:
    - type: gelf
      facility: FACILITY
      host: HOST
      includeFullMDC: true 
```

Cheers,
Michal