Allow for nesting of singleton belong_to's.

In reference to issue #141.

Note that

```ruby
class ThingAController < InheritedResources::Base
    belongs_to :thing_b, :singleton => true
end
```

used to mean there was only one `ThingA` (so `resource_path` == `/thing_bs/123/thing_a`), while it now means there is only one `ThingB` (so `resource_path` == `/thing_b/thing_as/123`)

To specify there is only one `ThingA`, you'll need

```ruby
defaults :singleton => true
```