Give isAllowed() the ability to check if the either the user's id or their assigned roles provide access to a resource

See issue: https://github.com/OptimalBits/node_acl/issues/76

When `isAllowed()` is called it will check if any of the user's permissions provide access to the resource, or if the user has been given access to the resource by using their userId as the role name.

This just avoids having to call both `isAllowed()` and `areAnyRolesAllowed()` to check is the user has access.