The way that getbalance works currently cannot check the balance of an individual address. However the whole accounts system in Bitcoin Core has been deprecated and is being removed, so you will probably be able to do that in a major release or two (after 0.14).
The accounts are the same as the Labels that you see in the Receiving Addresses list. By default, the account is blank, so you would type
in order to get the balance of all of your wallets.
For one address, you could assign it its own account by changing the label and callling
where <account> is the label you gave the address.