support rest backends that do not expect an id

this adds a new functionality to support rest endpoints that doesn't expect an id in the request urls. to accomplish this new optional feature a third parameter has been added to `one`.

an example:

```javascript
Restangular.setBaseUrl('http://google.com')

// default behaviour
Restangular.one('user', 1).getRestangularUrl()
// -> "http://google.com/user/1"

// new optional behaviour
Restangular.one('result', 1, true).getRestangularUrl()
// -> "http://google.com/result"
```

we need this as we are using a rails backend with single resource controllers. requests to this controllers that include an id actually get rejected.

```ruby
# routes.rb [...]
scope module: 'user' do
  resource :address, only: [:show, :create, :update, :destroy]
end

# produces..
# POST    /latest/users/:user_id/address(.:format)
# GET     /latest/users/:user_id/address(.:format)
# PATCH   /latest/users/:user_id/address(.:format)
# PUT     /latest/users/:user_id/address(.:format)
# DELETE  /latest/users/:user_id/address(.:format)
```

also, when you bind to `Restangular.one('result', 0, true).get().$object` I enforced the object initialized to `$object` to adopt the same single resource behaviour.

I also did add tests for this. however, i changed (actually removed) parts of the implementation I even think it was wrong code. the tests didn't complain about my change. but maybe someone who is more into this should have a look. i'll add a note on that line.

cheers,
johannes.