Update Alert.php to display multiple flash messages for the session.

Cleaned up a few comments ... hope its not my OCD again... :-)

Reverted name to ```closeButton``` to be in sync with the bootstrap ```Alert``` widget.

1. Renamed closeButton to closeButtonOptions. 
2. Generate unique id for each alert box.
3. Moved parent init to the top.

Include Schema in new migrations by default

fixes #1261

worked though all the guide files adjusted structure and
added some missing does

Fixes #1222: removed Slider widget from JUI extension since it's not input widget and a wrapper doesn't really add anything

Changed usage of self to static to allow extendibility

fixes #1244

fixes #1246

Each flash variable can be mapped to a bootstrap alert type through a configuration array passed to this widget.

code style section has been added here:
https://github.com/yiisoft/yii2/wiki/Core-framework-code-style#self-vs-static

verification code for properties:
https://gist.github.com/cebe/7566916

Cleaned up redundant code (and a typo missed earlier).

Removed redundant declarations and checks in the code.

Ensure previous CSS classes are not appended and reinitialized in the foreach loop.

issue #1246

also made everything conistent with client validation

Used ```Html::addCssClass``` and ```Html::removeCssClass``` more effectively.

Added closeButton parameter, assignment of CSS class corrected, and added php doc comments.

this allows to be able to call error action.

fixes #1244

fixes #1220

The previous alert widget within the advanced/basic app does not render multiple flash messages. The current implementation allows display of all flash messages assigned through setFlash. Also a validation is done for allowed bootstrap alert types.

Small edit for consistency with frontend config.