[WIP] New asset management

Fixes https://github.com/yiisoft/yii2/issues/4454

## Current status

Basic app can work after running the following bower commands manually:

```
bower install yii2#master --save
bower install yii2-bootstrap#master --save
bower install yii2-debug#master --save
```

## Tasks to be done

- [ ] fix `yii2-advanced-app`
- [ ] fix `yii2-jui` extension
- [ ] fix `yii2-authclient` extension
- [ ] support automatic bower package installation (via bowerphp) when using composer to install an extension
- [ ] enhance `asset` command to support asset bundle listing and asset combining/compression via grunt and other tools (may postpone this task to 2.0.1)
- [ ] documentation

I like the change. Syntax is now more explanatory.