start to track performance evolution

[ichiriac]

related to a discussion on #455 - @czosel you are welcome to contribute on this :

• We could use benchmark.js framework as the results may be more accurate
• Add a traking JSON test/perf/statistics.json
• At each run lookup if there is a result entry
• Scan the test/perf folder for php files, and on each run tests on tokens & nodes (and check the threshold)
• If entry found, check with the threshold if the performance is ok
• If no entry, at the end of tests, create one and update the JSON file
• Add more tests to cover various use cases (but keep them generic enough to be run on php7.0)
• Checkout previous release tags and try to get performance statistics on each in order to see if performance has been degraded

[ichiriac]

related to #57

[coveralls]

Coverage remained the same at 96.143% when pulling 492bcfb on performance_tracking into 407e4d4 on master.

[czosel]

Landed in #885