serve():
  - only compress based on mime type using npm compressible version 1.0.1
  - re-write HEAD support to work with compressed content
  - cache gzip and deflate content
  - setHeader( 'Vary', 'Accept-Encoding' ) for caches
  - get content in a lazy fashion, using get_content() file method if provided
  - add text 'Not Modified' to 304 responses