Register built-in models in a standalone file

Move the code registering built-in models to a new file
`lib/builtin-models.js`.
Dismantle `lib/models`.

 - Move core models `Model` and `PersistedModel` to `lib/`.
 - Move `AccessContext` class to `lib/`, since it is not a model.
 - Move all other built-in models to `common/models`.

This is a preparation for extracting model definitions to JSON files.
By splitting the change into multiple commits, git is able to keep track
of file moves (renames).
Merge pull request #640 from strongloop/feature/move-models-to-common

Move models to common/models