Add createModelFromConfig and configureModel()

Add new API allowing developers to split the model definition and
configuration into two steps:

 1. Build models from JSON config, export them for re-use:

  ```js
  var Customer = loopback.createModelFromConfig({
    name: 'Customer',
    base: 'User',
    properties: {
      address: 'string'
    }
  });
  ```

 2. Attach existing models to a dataSource and a loopback app,
    modify certain model aspects like relations:

  ```js
  loopback.configureModel(Customer, {
    dataSource: db,
    relations: { /* ... */ }
  });
  ```

Rework `app.model` to use `loopback.configureModel` under the hood.
Here is the new usage:

```js
var Customer = require('./models').Customer;

app.model(Customer, {
  dataSource: 'db',
  relations: { /* ... */ }
});
```

In order to preserve backwards compatibility,
`app.model(name, config)` calls both `createModelFromConfig`
and `configureModel`.
Remove assertIsModel and isDataSource

Use `instanceof` operator instead:

  ModelCtor.prototype instanceof loopback.Model
  dataSource instanceof loopback.DataSource
refactor: extract runtime and registry

Move isBrowser and isServer from lib/loopback to a new file lib/runtime.

Move all Model and DataSource related methods like `createModel` and
`createDataSource` to lib/registry.

Remove the circular dependency between lib/application and lib/loopback,
by loading lib/registry and/or lib/runtime instead of lib/loopback
where appropriate

This commit is only moving the code around, the functionality should
not be changed at all.
test: add debug logs

Add debug logs to troubleshoot two unit tests failing
on the CI server only.
lib/registry fix jsdoc comments

Add missing names.
Merge createModelFromConfig with createModel

Merge the two methods `loopback.createModel` and
`loopback.createModelFromConfig` into a single method `createModel`.
registry: fix non-unique default dataSources

Fix the problem where `registry.defaultDataSources` has two instances:

 - `require('loopback').defaultDataSources` used by
   `loopback.autoAttach()`

 - `require('./registry').defaultDataSources` used by
   `app.dataSource`.

I am intentionally leaving out unit-tests as the whole `autoAttach`
feature is going to be deleted before 2.0 is released.
registry: export DataSource class

Expose the juggler's DataSource constructor as `loopback.DataSource`.

The DataSource constructor is most useful to check
for `instanceof DataSource`, but it also makes the loopback API more
consistent, since the API is already exposing all pre-built Models.