Refactor todo controller to serialize JSON properly (fixes #3)

This PR adds the `inherited_resources` gem, which effectively removes much of the current controller. 

When I was saving a todo, I got the error that it was sending the key `id`, but I didn't have a mapping for it. It turns out the JSON that was returned was this:

```json
{id:66, title:try this, is_completed:false}
```

But Ember expected there to be a root element, like so:

```json
{todo: {id:66, title:try this, is_completed:false}}
```

I began by looking at the serialization. I changed attributes, and Rails didn't seem to be using it. I noticed that the serializer in this project was called `TodosSerializer`, while Rails was looking for a serializer called `TodoSerializer`, in the singular, like the model. (See [this section of the AMS docs](https://github.com/rails-api/active_model_serializers#render-json). So I changed that, but the error still didn't go away.

I knew from previous experience, however, that `inherited_resources` and `active_model_serializers` play really nicely together. I had remembered from another project that it used `inherited_resources` with Ember, and I didn't remember getting that JSON serialization error.

Thus after adding, `inherited_resources` (and deleting most of the controller code for free in the process), the issue was fixed. I tried to dig into the source to figure out why it wasn't working before, but `inherited_resources` is pretty tough to figure out. So at the end of the day, this fixes #3.

I recognize that it adds a new gem dependency, but I felt it was a worthwhile trade-off, given how much code it allowed to be deleted.