Add streaming stats to bolt bench.

This commit adds -stats-interval to the 'bolt bench' utility. By setting this argument to an interval greater than `0s`, the benchmark tool will output stats as streaming JSON. This data can, in turn, be graphed to understand performance over time.

Note: This field is simply parsed by [`time.ParseDuration()`](http://golang.org/pkg/time/#ParseDuration) so see that function's format for more details.

Note: The final timings are now printed to `stderr` so that the `stdout` can be output cleanly and consumed by another charting application.

/cc @snormore @mkobetic