Added fixture classes.
removed trash code.
Added FixtureTrait.
finished fixture feature.
improved doc.
Added tests for Fixture feature.
refactored fixture implementation.
doc fix.
Update Migration.php

add ability to use an array in multi columns parameters
Merge pull request #2039 from dizews/patch-1

Update Migration.php
Removing `abstract` in Module
Merge pull request #2044 from slavcodev/patch-1

Removing `abstract` in Module
refactored fixture.
doc fix
Merge pull request #2050 from yiisoft/feature-fixture

Feature fixture
updated change log [skip ci]
Fixes #2051: Do not save null data into database when using RBAC
AutoTimestamp field renaming
Advanced app User model updated
Advanced app migrations updated