Add function to assist with rebalancing

This function takes either a database name or a list of shards and a
list of target nodes to balance the shards across. Every node with
less than a fair share of shards will steal shards from the node with
the most shards as long as both shards are in the same zone.

BugzID: 18638
Merge pull request #45 from cloudant/18638-balance-function

Add function to assist with rebalancing