Change 'defaultAttributes' to 'defaultComponents' in Primitive Definitions

[msimpson]

This needs review as I had to alter the prototype to use defaultComponentsFromPrimitive in registerPrimitive instead of simply defaultComponents since Object.create will override the defaultComponents from the a-entity prototype. This, if allowed, would cause test failures as getAttribute subsequently returns null for those overridden attributes (i.e. position, scale, etc.).

Fixes #1330.

[dmarcos]

It looks good to me. I let @ngokevin merge it.