API spec refactoring and extension

* Rearrange specs into different files for different resources
* Refactore specs to be closer to community practices (though not entirely ["best practices"](betterspecs.org), yet)
* Change expectation for `tasks#create` -- created `Task` was not being assigned to the `Checklist` the request was made for
* Add spec for `tasks#index` (ie, I want to be able to make an API request to see all the tasks in my checklist; was not previously able to)
* Add spec to only allow authenticated requests for `checklists#create` -- use this as a guide for adapting other request specs.  For hints on implementation, see [Devise github wiki](https://github.com/plataformatec/devise/wiki)