PSR-4 Autoloader

Hi!

I've recently found that Codeception doesn't support neither PSR-0 nor PSR-4 autoloading for its Helpers, StepObjects etc. So I took example autoloader with tests from [here](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-4-autoloader-examples.md).

Then I added support for empty namespace prefix, so one can use `Autoloader::addNamespace('', 'path')` to load `Foo\Bar\ClassName` from `path/Foo/Bar/ClassName.php`.

Finally, this autoloader is backward compatible with an old autoloader, so all existing code should work as before.

**Old default autoloading:** all classes with `Helper` suffix from `helpers` dir. Namespace doesn't matter, because Autoloader doesn't look in subdirectories.

**New default autoloading:** according to PSR-4, all Helpers in global namespace and those in `Codeception\Module` one will be autoloaded. Basically, it's done in runtime like this:
```php
// paths.helpers from codeception.yml
Autoloader::addNamespace('', 'path/to/helpers');
Autoloader::addNamespace('Codeception\Module', 'path/to/helpers');
```

The only issue users might face with strict PSR-4 autoloader is that classes in custom namespaces wouldn't be autoloaded, i.e.:
```php
Autoloader::registerSuffix('Helper', 'path'); // in existing code
// is now equal to this
Autoloader::addNamespace('', 'path');
// so if Foo\Bar\ClassName is located in 'path/ClassName.php',
// it will NOT be autoloadable via PSR-4 rules
$a = new Foo\Bar\ClassName; // Autoloader will look for it in 'path/Foo/Bar/ClassName.php'
```

That's why in case of missing file for `Foo\ClassName` using strict rules of PSR-4, this autoloader also tries to load `Foo\Bar\ClassName` from `path/ClassName.php`, just like an old autoloader did.