Add $layout parameter to Observer::notify('page_before_execute_layout') 

Observer page_before_execute_layout was a bit useless without parameter. Adding the $layout variable to Observe::notify() makes it possible to **do something** with layout code before actual **execution**.

This can be used for example to inject some markup into any layout regardless of it's code.

A sample usage based on code of Skeleton plugin:

```php

Plugin::setInfos(array(
    'id'          => 'skeleton',
    'title'       => __('Skeleton'),
    'description' => __('Provides a basic plugin implementation. (try enabling it!)'),
    'version'     => '1.1.0',
   	'license'     => 'GPL',
	'author'      => 'Martijn van der Kleijn',
    'website'     => 'http://www.wolfcms.org/',
    'update_url'  => 'http://www.wolfcms.org/plugin-versions.xml',
    'require_wolf_version' => '0.5.5'
));

Plugin::addController('skeleton', __('Skeleton'), 'admin_view', false);

Observer::observe('page_before_execute_layout', 'pbel_handler');

// adds test in front of every page
// and end of test at the end
function pbel_handler( $layout ) {
    $layout -> content = 'test ' . $layout -> content . ' end of test';
}

```

PS. My netbeans automagically trims whitespace from line endings so commit looks a bit confusing, sorry