Rework models to use the new 2.0 layout

Split model definition and model configuration.

`models/` is a require()-able component providing all shared models.

`html5/local-models` is another require()-able component providing
html5-specific models like LocalTodo.

`api/models.json` and `html5/models.json` contain data-source
configuration for the loopback apps.

Modify package.json to track github branch 2.0
of both loopback and loopback-testing.