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

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.
Update Alert.php
Update Alert.php to add some refinements and clean up bugs.

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

Used ```Html::addCssClass``` and ```Html::removeCssClass``` more effectively.
CSS Class assignment correction # 2.

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

Removed redundant declarations and checks in the code.
Code & minor bug cleanup.

Cleaned up redundant code (and a typo missed earlier).
Changed usage of self to static to allow extendibility

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
Alert types for flash messages are now configurable.

Each flash variable can be mapped to a bootstrap alert type through a configuration array passed to this widget.
reverted duplicated fix for #1244

fixes #1244
Added warning log when access to gii and debugger is denied due to IP restriction.
Merge pull request #1258 from cebe/self-static

Changed usage of self to static to allow extendibility
Fixes #1245: LinkPager now renders disabled links as spans preventing these to be clickable
Fixes #1255: typecast empty string to null for non-string DB columns.
Fixed public property naming.
Fixes #1222: removed Slider widget from JUI extension since it's not input widget and a wrapper doesn't really add anything
Added assets section to the guide
restructuring guide index.md and structure

worked though all the guide files adjusted structure and
added some missing does
updated application guide and index
more on yii basics