Finished SecurityHelper.

- Refactored checking for fatal error.
- Suppressed errors in exception handler in case of fatal errors.

- Correct exit code for require errors in CLI mode. It was 0 even in case of error.
- Solved handling fatals and displaying custom error template w/o output buffering.
- If XDEBUG is available, error screen will display trace (implementation inspired by Kohana and Nette).
- Added ErrorException to store/display type of the error and user-friendly messages.
- Fatals are still logged in PHP log.
- Turned off native display_errors since we're now catching all errors.
- Added reserving memory (256kb) . Can be removed safely. In case of removal we're losing only memory exhausted error when allocating last very small chunk of memory.
- Properly handled errors in __toString (exception can't be thrown in this case).
- In the YII_DEBUG===false mode it's bad to display exception name even in page title.
- In the YII_DEBUG===true mode it's still useful to get user-friendly message additionally to exception name.