Convenience Methods for Account Stores

@lhazlewood Hey Les. The `AccountStoreMapping addAccountStore(AccountStore accountStore);` method already exists. However, it requires the `accountStore` to have the `href`; it does not do any lookup based on the `name`. The implementation I wrote for both methods (`addDirectory` and `addGroup`) invokes the `addAccountStore(AccountStore)` method after looking for the desired resource. I agree with you that these two methods seem redundant. I will modify the existing `addAccountStore(AccountStore)` method since this change will not break anything for any existent code, it will add more flexibility to its behavior. The only thing that will be broken are tests that expect some exception if the `href` is not provided (this could impact customers if they have such tests). Now, no `Exception` will be thrown if the `href` is not provided but the `name` is.