Add option --with-score to summary.py to automatically classify speedups

Legend for new column is:
* FASTER - strong speedup
* faster - spedup is detected but it is unreliable
* <empty> - no change in speed
* slower - slowdown is detected but it is unreliable
* SLOWER - strong slowdown
Merge pull request #563 from taka-no-me:perf_classify_speedups