Ground Path Through battery cells
A former colleague of mine had devised an efficient way to solve the cell balance issue by transferring charge from one cell to another by switching capacitors. You'd probably want to add some current limiting to avoid excess currents between the capacitor and battery cells that would either present a hazard to the battery in the absence of a safety circuit or result in blowing fuses or tripping safety circuits in the presence of a safety circuit. The idea is patented and can be found here https://www.google.com/patents/US6624612 so you would need to be mindful if using this technique in a commercial setting.

If you're just looking to get the job done, I'd recommend purchasing an off the shelf multi cell battery management solution. TI, Linear Tech and others make silicon for these applications. Their solutions often also include safety and fuel gauging built in. http://www.ti.com/lsds/ti/power-management/battery-monitor-protection-and-authentication-solutions-overview.page