Added `yii\composer\Installer::postCreateProject()` and modified the syntax of calling installer methods in composer.json [skip ci]

- Moved everything test-related into `tests` directory. Codeception tests are in `codeception`.
- Removed database module since we're using fixtures and migrations.
- Moved console entry points and bootstrap into `tests/codeception/bin`.
- Adjusted travis build scripts.
- Adjusted documentation to be consistent and reflect changes made.

[ci skip]

Fixes #4597: `yii\composer\Installer::setPermission()` supports setting permission for both directories and files now (qiangxue)