Benchmark results: store in a CSV file, graph as SVG

[aklomp]

Benchmark results are currently "stored" in a big table in the README which is getting ungainly and hard to modify. The results are presented in text form, which makes it harder to compare across machines and codecs.

Objectives:

• Move the benchmark results to a CSV file (or comparable textural data format) in tests/benchmarks.
• Write a script (or use Gnuplot, etc) to autogenerate a SVG chart from the results. Automate the steps in a Makefile. The chart should probably be a horizontal bar graph with a bar cluster for each machine.
• Link the SVG chart in the README as an inline image.