- update composer json to chosse right path
- set vendorPath in application config if not default

issue #418

Moved add prefix ($this->keyPrefix) to the key in function buildKey(). Could it be better?

conflicting css class and bad positioned icon

`$this` -> `$self`

Fixed an example of using buildKey in the doc

PHP supports exception stacks since 5.3 so we should use it.
Also Errorhandler is now able to display the stack: #297

make sure all information is displayed
Fixes comment by @creocoder issue #408
https://github.com/yiisoft/yii2/pull/408#issuecomment-18454179