Add note for config COW as proxy for all protocol.
add support to ip range and limit remote to ipv4 address only

proxifier would pass ip address to proxy with "Resolve hostnames through
proxy" turned off
add support for CIDR
remove builtin sites (which are domains)
remove PAC server
build script: use gnu-sed and build for darwin amd64 only
update sitestat.go

1. set siteStaleThreshold to 3 days
2. do not remove outdated stats
3. keep blocked sites in stat only
decrease maximum number of connections to a server
decrease hard-coded readTimeout and dialTimeout