urlserver.py: subdirectory support and small fixes

- make it possible to run reverse proxy in a subdirectory by
  generating relative links and using the <base> tag. to use this, set
  http_hostname_display to 'domain.tld/subdir'.
- mention favicon explicitly (now works in subdirectories, too).
- update favicon to new weechat logo.
- set meta referrer to never in redirect page, so chrome users'
  referrers are hidden, too
- fix http_auth in chrome and other browsers which send header names
  in lower case
- bump version to 1.3