Add missing CPU field, change field length check

Fixes #54
Set var for rate type
Get rid of metric variable, get rid of continue
Default send location to http://bosun/

Optionally disable metadata sending. This is work to allow bosun to stop
relaying :4242 traffic.
Don't fatal on bad external tag

fixes #62
More verbose bad tag errors
Return, don't print, scan error
If /sys/class/net/if/speed exists and is readable, use it to populate the linux.net.ifspeed and os.net.ifspeed metric