add disk IoTime on freebsd and fix read & write time calculation

[phemmer]

This PR does 2 things.

1. It adds the IoTime metric to FreeBSD
2. It fixes the the disk time calculations.

In regards to number 2, the Compute() function was effectively doing nothing. It was converting BINTIME_SCALE to a uint64. But because the value is a float < 0, when converted to int64, it becomes 0. So the function effectively just returned the field b.Sec. This fixes the calculation by preserving the value as a float.
Also relatedly, the times were being returned in seconds, when the raw values offer much higher precision than that. Since the linux values are returned in milliseconds, I opted to keep the type (a uint64), but am now returning the values measured in milliseconds instead of seconds. (It would probably be a good idea to use a time.Duration instead of uint64)

[shirou]

Confirmed on my FreeBSD 10, great contribution. Thanks a lot!

(It would probably be a good idea to use a time.Duration instead of uint64)

Hmm, you're partly right. However, I want to avoid API change, and convert time.Duration from milliseconds is easy. so, I prefer it keeps.