Workaround PHP bug 42098, autoload in strict errors

_Previously https://github.com/filp/whoops/issues/69._

[PHP bug 42098](https://bugs.php.net/bug.php?id=42098) prevents autoloading Whoops classes on strict warnings.

As a result, instead of seeing a strict warning, a user of Whoops library sees `Class \Whoops\Exception\ErrorException does not exist` without any traceback, because it is a fatal error.

Given that PHP devs, as usual, have marked the bug as "Not a bug", I suggest to add a tiny workaround to Whoops to pre-autoload required classes to handle a strict warning:

    class_exists('\\Whoops\\Exception\\ErrorException');
    class_exists('\\Whoops\\Exception\\FrameCollection');
    class_exists('\\Whoops\\Exception\\Frame');
    class_exists('\\Whoops\\Exception\\Inspector');