Configure USB drive to spin down automatically

Mostly for power consumption, but basically—make this a configurable option (e.g. either enable it or disable it via a bool), and also allow configuration of the time before the drive spins down after inactivity.

Some people might use an SSD here, but most people would likely toss an old spinning rust drive at the Pi for backups (since it's expendable... and since the Pi's USB 2.0 bust can't sustain super fast transfer, at least not in the current generation). So it's nice to conserve energy. Just don't use too quick a frequency for the default spindown as frequent toggles can cause the drive to wear faster.

For some references:
- http://superuser.com/a/937497/80658
- https://www.raspberrypi.org/forums/viewtopic.php?f=63&t=6070#p202105
- http://www.nslu2-linux.org/wiki/FAQ/SpinDownUSBHarddisks