Move the fixture.text file to a fixtures folder

As we may want to have other fixtures for tests, let's create a proper
folder containing all of them.
Use the benchmark-ips gem for benchmarking

Instead of relying on the provided benchmark module, let's rely on the
benchmark-ips gem.

Since Kramdown is slow (~ 6 minutes to parse 10 thousands time the same
document), this is now a lot more easy to have feedback plus the
feedback is more useful because we have the amount of variation.
Merge pull request #387 from vmg/benchmark-improvements

Benchmark improvements