Graphite suffix

In our use of statsd (which we run on every host) & graphite we send the hostname as the last component of the stat, like this:

```
<environment>.<appname>.<key>.<host>
```

And roll that up using the aggregator like this:

```
<environment>.counters.<key>.total_<type>_avg (10) = avg <prefix>.counters.<<key>>.<host>.<type>
<environment>.counters.<key>.total_<type>_sum (10) = sum <prefix>.counters.<<key>>.<host>.<type> 
```

In the current version of Statsd, it means that every app must remember to add the hostname at the end of every stat it's sending.

It'd be a lot easier if we could add this to Statsd as a setting.

This pull request does exactly that, and has tests & documentation attached.