Convenience Methods for Account Stores

This method and the `addGroup` should be refactored to a single method:

`AccountStoreMapping addAccountStore(AccountStore accountStore)`;

This is polymorphic; both `Directory` and `Group` implement the `AccountStore` interface.

Also `addGroup` could be confusing because it is not obvious if that means 'add a group to the Application's total collection of groups' or 'add a group to the Application's account stores'.   `addAccountStore` is explicit.