Fix ChromePhp logger interface and debug level

The `ChromePhp` log writer logs debug messages as error messages. It looks like the interface was copied from `FirePhp` and not updated.

This pull request changes the `ChromePhpInterface` to match the [actual class](https://github.com/ccampbell/chromephp/blob/master/ChromePhp.php) and fixes the log level used for debug messages.