``redis`` basic installation

[Mischback]

rspamd relies on redis as a fast caching backend.

This issue is directly related to #15.

Requirements

• install redis from Debian/stable
• provide a basic configuration
  • only available locally (bind to localhost)
  • limit RAM usage (needs investigation)
  • write to disk frequently (needs investigation)
• Can redis be used for other required caching?
  • Postfix?
  • nginx
  • Django