Add view.register method

This helps us avoid leaking views that are instantiated but never inserted into a child view, thus never destroyed. Applications that manually insert views into the DOM will now need to be sure to call `register` or `_addChild` if they also need to use `$.view`.

Fixes #363
Move register method into retain

This simplifies the external API as register was basically a form of retain.
Merge pull request #364 from walmartlabs/register-view

Add view.register method