Instrument model, rendering etc. code using performance.js module
Refactor MD2D benchmarks completely

Now data is collected during single run to ensure that all results
correlate better.

Use performance.js to get time spent in various scopes.

Additional changes:
- present time of various events in "ms" instead of "steps / s"
- rename some measurements (e.g. graphics is now JS rendering),
- add "tick" measurement, which consists of model step, JS rendering
  and all other JS code (e.g. user scripts). This is *not* simple sum
  of "model" and "JS rendering" values.