Add proper logging to router

See flynn-archive/strowger#25.

This is still a work in progress, but I'm adding it here for early feedback. The output looks like this:

	DBUG[09-02|14:33:48] New client connection                    module=http conn=0xc2100aa9b0 raddr=[::1]:63102 tls=false
	DBUG[09-02|14:33:48] Reading next request from client         module=http conn=0xc2100aa9b0 raddr=[::1]:63102
	DBUG[09-02|14:33:48] Begin processing request                 module=http conn=0xc2100aa9b0 raddr=[::1]:63102 req=0xc210000d88 srv=tumbor
	DBUG[09-02|14:33:48] Select backend for service               module=http conn=0xc2100aa9b0 raddr=[::1]:63102 req=0xc210000d88 srv=tumbor
	DBUG[09-02|14:33:48] Dialing the backend                      module=http addr=docker:8888
	DBUG[09-02|14:33:48] Starting I/O for request                 module=http conn=0xc2100aa9b0 raddr=[::1]:63102 req=0xc210000d88 srv=tumbor
	DBUG[09-02|14:33:48] Backend returned response                module=http conn=0xc2100aa9b0 raddr=[::1]:63102 req=0xc210000d88 srv=tumbor
	DBUG[09-02|14:33:48] Done processing request                  module=http conn=0xc2100aa9b0 raddr=[::1]:63102 req=0xc210000d88 srv=tumbor
	DBUG[09-02|14:33:48] Reading next request from client         module=http conn=0xc2100aa9b0 raddr=[::1]:63102