Use new_resource in new action

Created by: EugeneGrigorenko

• Simplify making new form with prefilled associations
• Is supposed to close this issue https://github.com/thoughtbot/administrate/issues/220
• Now to have simply override new_resource method instead of new action to init resource associations

P.S. Though, it works as expected on has_one association, when trying to build has_many association it doesn't work. The reason is that select is used on has_many form and only ids get send with the request. And because the record doesn't have an id it gets lost. I'm not sure if it's a bug and should be addressed or not, please let me know what you think.