Initial implementation

Move `app.boot()` and its tests from loopback.

Fix jshint warnings.

Clean up unit tests - remove dependency on global variables created
by loopback's test/support.js
Load datasources and app cfg from multiple files

Modify loading of `appConfig` and `dataSourceConfig` to look for
the following files:

  - app.json
  - app.local.{js|json}
  - app.{$env}.{js|json}

  - datasources.json
  - datasources.local.{js|json}
  - datasources.{$env}.{js|json}

where $env is the value of `app.get('env')`, which usually defaults
to `process.env.NODE_ENV`.

The values in the additional files are applied to the config object,
overwritting any existing values. The new values must be value types
like String or Number; Object and Array are not supported.

Additional datasource config files cannot define new datasources,
only modify existing ones.

The commit includes refactoring of the config-loading code into
a standalone file.
Custom rootDir for models and datasources

Support custom project layouts where model & datasource config files
are located in a different place from the app config.

Example:

    # API server
    server/app.config.json
    server/datasources.json

    # shared between server & client
    models.json
    models/

    # isomorphic client
    client/app.config.json
    client/datasources.json
cleanup: remove unused code in requireDir()

No changes in functionality.
Pass `app` to fn exported by auto-required script

When a script in `models/` or `boot/` exports a function which is not
a loopback.Model constructor, the bootstrapper immediatelly calls
this exported function wit the current `app` object.

This is providing a dependency injection mechanism for boot scripts,
so that they no longer need to know where to find the `app` object.

Note: the dependency injection is optional. Existing code getting
`app` reference via `require('../app')` will continue to work.
Start CHANGES.md
Ignore models/ and boot/ subdirs without index

Sub-directories of `models/` and `boot/` that cannot be required
(they don't have an index.js file) are silently skipped now.

This enables developers to put test files into `models/test/`.
fixup! change debug logger name to 'loopback:boot'
Merge pull request #1 from strongloop/feature/extract-app-boot

Initial implementation