Do not use Whoops if YII_DEBUG is false or if errorAction is set and exception is CHttpException.

The latter allows you to work on and test your error action in development.
Correctly skipping useless exceptions in case of PHP errors and leaving stack untouched in case of real exceptions
Respecting debug and improving further handlers

*This is a merge of ideas from #6 (nkovacs/yii-whoops@60d4f99) into this repo,
with a different implementation.*

- Debug behaviour from `CErrorHandler` was copied into `WhoopsErrorHandler`,
  respecting error pages in production, ajax calls, and so on
- Originally I was using `Yii::app()->end()` in the `errorAction` to explicitly
  stop Whoops handler, but that's a foggy API. Thus, it was added a `handled()`
  method to be called inside `errorAction` to make it clear `WhoopsErrorHandler`
  should stop working. This clearly enables a last-resort error handling
  implementation, so you can "catch" application-wide issues and still leave
  space for Whoops to show error pages for those you didn't handled.
- Added information on those API changes to README, plus information on the new
  version of `Yii::app()->errorHandler->error`.
Reverting nkovacs commit for some reasons

There was parallel development between his fork and the original repo,
and thus the merge would not happen anyway. So I decided to pull in his
changes into that part of the history (to still refer his work) and
implement the ideas in a different way, that would anyway fit better
with the project's ideas.
Merge branch 'master' into erroraction