benchmark

Add a "compare_bench.py" tooling script. (#266)

This patch adds the compare_bench.py utility which can be used to compare the result of benchmarks.
The program is invoked like:

$ compare_bench.py <old-benchmark> <new-benchmark> [benchmark options]...
Where <old-benchmark> and <new-benchmark> either specify a benchmark executable file, or a JSON output file. The type of the input file is automatically detected. If a benchmark executable is specified then the benchmark is run to obtain the results. Otherwise the results are simply loaded from the output file.