Allow valid JSON output from benchmark

Created by: aprokop

The PR contains the patches to produce valid JSON output. The functionality of the driver was slightly changed. It no longer produces any neighbor statistics output, and version and hash were made optional by introducing a --no-header command line option.

I consider this to be the simplest way to address the problem. Other approaches are more involved. Ideally, the version/hash output would depend on the --benchmark_format=json option, but that would require more intimate interaction between boost program options and benchmark options parsing.