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