benchmark: add plot_csv R graphing script

This commit adds a graphing script (in R) for graphing the CSV output
of a benchmark. It can be run like this:

```
$ OUTPUT_FORMAT=csv iojs benchmark/http/client-request-body.js >
data.csv
$ ./benchmark/plot_csv.R data.csv graph.png bytes type
```

This will graph the output to `graph.png`, using the output's `bytes`
value as X and the result value for each as Y. Output will be grouped
by `type`.

Running as the example yields a beautiful graph like this:
http://pbrd.co/1vBhUfy.

PR-URL: https://github.com/iojs/io.js/pull/777


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>