Have 'getpeerinfo' show the peers' time offsets relative to our own.

This should be useful for debugging time issues in the future.

Example:

    {
        "addr" : "a.b.c.d",
        "services" : "00000001",
        "lastsend" : 1417333771,
        "lastrecv" : 1417333808,
        "conntime" : 1417333770,
        "pingtime" : 0.86318900,
        "version" : 60014,
        "timeoffset" : -2, <-- this is new
        "subver" : "/Satoshi:1.4.3/",
        "inbound" : false,
        "startingheight" : 230992,
        "banscore" : 0
    }