This commit adds an `OUTPUT_FORMAT` environment variable option for
all benchmark tests that allow either 'csv' or 'default' output. Default
output has been left unchanged, and csv output prints out the csv
headers along with the csv formatted per-test output, each test also
seperated by a newline.

It can be used like the following:
$ OUTPUT_FORMAT=csv iojs benchmark/common.js http

Not specifying the OUTPUT_FORMAT env var will default to 'default'.
Specifying a bad value will throw an error.

This commit adds an `OUTPUT_FORMAT` environment variable option for
all benchmark tests that allow either 'csv' or 'default' output. Default
output has been left unchanged, and csv output prints out the csv
headers along with the csv formatted per-test output, each test also
seperated by a newline.

It can be used like the following:
$ OUTPUT_FORMAT=csv iojs benchmark/common.js http

Not specifying the OUTPUT_FORMAT env var will default to 'default'.
Specifying a bad value will throw an error.

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.