move `plugin.php` to `Classes/Plugin.php`
provide patch for Phile-v1.x

This patch replaces the `phileCustomPostPreview` class by `Plugin` and use it
for the namespace instead.

Also explicit use of global namespace for `Twig_SimpleFilter`, because the
plugin is scoped in its namespace.
installation instructions for Phile-v1.x
Merge pull request #3 from Stijn-Flipper/patch-phile-v1.x

Add phileCMS v1.x support