Duplicate APC requirement removed

issue #1228

* 'master' of github.com:yiisoft/yii2:
  Edited model.md up to Validation
  Just started editing

It is more obvious what to pass in trigger() method

JUI readme: Clarification about clientOptions and using JUI in AR form

I accidentally made this pull request in the read-only JUI repo, so I'm doing it again here.

 [WIP] Caching: mset() method to store multiple items.

Must be one method to get _GET. 

Made ActiveRecord::isPrimaryKey() public

issue #1207

Eliminating $_GET array

+ added tests and fixed behavior