reduce log level of some common operations

our logs are flooded with cache messages,
which we generally only care about when they are slow.

This demotes most of these to debug-level unless they are slow.
fix debug demotion in log_handler

any 304 should be debug-level, not just static files
add favicon.ico and robots.txt
Merge pull request #233 from minrk/less-logging

reduce log level of some common operations