compiler: Move model-sources cfg to models.json

Remove `modelSources` option from `boot()` options, add `_meta.sources`
to `models.json`.

```json
{
  "_meta": {
    "sources": ["./custom/path/to/models"]
  },
  "Car": {
    "dataSource": "db"
  }
}
```
executor: Split model boot into two phases

In the first phase, all models are defined.

In the second phase, models are configured, attached to data-sources
and exposed on the app object.

This way when the `attached` Model event is emitted, all models are
already defined and thus a listener can get reference of any other
model used in the app.
compiler: Sort models topologically

Sort models topologically using Base->Model as edges. This way
the base models are defined before the models extending them.
Merge pull request #11 from strongloop/model-boot-improvements

[2.0] Model boot improvements