Add automatic removal of any previous output buffering.

Hi @filp, this is what I had in mind [here](https://github.com/filp/whoops/issues/33#issuecomment-16573928).

Might be good if @taylorotwell had a peak to make sure it doesn't mess up his Laravel 4 [implemenation](https://github.com/laravel/framework/blob/64f3a79aae254b71550a8097880f0b0e09062d24/src/Illuminate/Exception/ExceptionServiceProvider.php).