fix template guide twig section

e.g. UserEvent::trigger('event', 'class');
should result in a UserEvent to be passed to the handlers instead of
a base\Event.

Redundant event's instanceof check and some docs punctuation

And some docs punctuation.

closes #1762

Basic app: DB configuration in separated file

fixes #1736

Added to the basic app, fixed config, added to core commands.

change was made in 0d263a12

issue #1744

issue #1744

migration guide improved

Some debug code style fixes...

Some debug code style fixes, and https://github.com/yiisoft/yii2/issues/1478#issuecomment-31446267 fix

Yii2 faker integration