better support for pod structure

Adding the `app/pods` directory makes it obvious that this structure is supported.
* In `app.js`, setting `podModulePrefix` enables resolving pods.
* In `tests/helpers/isolated_container.js` adding `podModulePrefix` enables isolated testing of components using the pod structure.
* With this setting, component tests pass if `app/components/pretty-color.js` is moved to `app/pods/pretty-color/component.js`.