Write log to STDERR when XPF_ACCESS_LOG is STDERR

Heroku captures stdout and stderr into app logs.

It would be simpler to write log to STDERR when XPF_ACCESS_LOG is not
set, but I'm concerned about compatibility.
Support Redis as cache storage
Set cache module to XPathFeed from app.psgi
Add .perl-version
Add cpanfile.snapshot
Add app.json

app.json is a manifest format for describing the way to deploy web apps.
With app.json, we can deploy web apps to Heroku by clicking a "Deploy to Heroku" button.