Add a simple stress task.
Cache _config.yml reads.
Cache ContentFile header retrieval.
For default post layouts, read/parse the file only once.
Update CHANGELOG about 29.
Support file_digest in rake stress.
Customisable iterations.
Rely on Benchmark for time measurements.

Avoids broken `stress` task on systems where `time` is a keyword.
Micro-cache Site#to_liquid for one second.

This causes a huge speed-up when using file_digest with ENV=production.

See #30.
Be more specific in CHANGELOG about performance.
Merge pull request #29 from aprescott/perf/caches

Improve performance of post generation with caches