Split out the CSV writing from basho_bench_stats

In an attempt to make stats writing modular, split out the CSV writing
parts into a separate file. This is only a refactoring, which should not
add, change or remove any functionality, but is a prerequisite of #147.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Add support for sending stats to Riemann

Add support for sending stats to Riemann via the Katja library.
Configuring Katja (to set defaults, such as host, tags and ttl) is done
via Application config.

This fixes #147.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Add a little documentation about the stats writer

Based on suggestions from Scott Lystig Fritchie, add some documentation
to the top of basho_benc_stats_writer.erl about using the different
writers, with copy & paste ready snippets.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge contrib-algernon/f/pluggable-stats to master