Auto-disable debugtoolbar depend of response type

This is a feature based on #58 Pull Request.

With this new feature, the debug toolbar will automatically disable itself whenever yii serves content that isn't on the white list.

This is useful for web services producing pdf, xml, csv, or json output, for serving binary data like images, ...

This should automatically solve issue #18 and maybe also issue #43 (depending on why one wants to disable the toolbar for certain routes).

And is better than #58 Pull Request because add validations of Whitelist types in the onBeginRequest method.