Pluggable stats & Riemann writer

This patch set consists of two parts: the first moves the stat writing code from `basho_bench_stats` to `basho_bench_stats_writer`, and makes it configurable with the `stats` setting (supporting CSV only). The second implements a stat writer that sends the various events to Riemann.

With both applied, setting `{stats, {riemann}}` will work. To configure where to send events, TTL and other properties, one will need an application config (which involves adding a `-config` argument to escript). I've been using the following application config for testing:

```erlang
%% -*- erlang -*-
[
  {katja, [
    {host, "127.0.0.1"},
    {port, 5555},
    {transport, detect},
    {pool, []},
    {defaults, [{host, "eowyn.local"},
                {tags, ["katja"]},
                {ttl, 5.0}]}
  ]}
].
```

This fixes #147.