Create performance tests to catch regression and memory leaks

[depili]

At Assembly Summer 2017 most of my on-event time was spent on hunting memory leaks. So we need to write some performance testcases to catch the problems and possible performance issues.

This is pretty much needed for issues #8 and #18

[depili]

Seems that the rails-perftest gem is just about useless for performance testing. Need to look elsewhere or implement the tests by hand.