Do not try include lang files if not present (avoid annonying messages in the logs)

Do not try include lang files if not present (avoid annonying messages in the logs)
It is now possible to pass Smarty specific configuration for templates (for now, assignments with $_tpl_assignments array in a _smarties.php file). Specials strings '__plugin_include_dir__' and '__plugin_templates_dir__' will be replaced by the relevant values before being sent to Smarty
If a file named 'header.tpl' exists in plugin_dir/{plugin_name}/templates/{current_theme} ; it will be loaded after galette's headers
Removed TinyMCE as mailing HTML editor, replaced by markItUp (TinyMCE will be available as a plugin)
Add a Smarty variable to check if default html editor should be used ($html_editor). Plugins that would replace markItUp should define this one. Anyways, I do not know what would happen if there are more than one html editor plugin installed