Adding a benchmak test structure to help contributors to keep track
of how their PR will impact overall performance.
It enables developers to create test inside of tests/benchmark.
This implementation adds a rake task: ```rake benchmark``` that checkout
one commit before, run the test of tests/benchmark, then mover back to
the last commit and run it again. By comparing the benchmark results between
both commits the contributor will notice if and how much his contribution
will impact overall performance.
