Add Megabit as a valid unit in metadata
If /sys/class/net/if/speed exists and is readable, use it to populate the linux.net.ifspeed and os.net.ifspeed metric
create variable for tag to make code easier to read
Remove error check on readLine since there's not much the end user can do about it anyway and for people who use Xen, it will produce an error every 15 seconds.
Merge pull request #63 from StackExchange/ifspeed_linux

add ifspeed metric to track interface speed