configured config and routing configuration files parsing them with Synfony2 Yml component
improved to automatically update config.yml assetic section and App.Kernel.php with the active theme, each time the website is deployed.

- refactored the .active_theme file to store both frontend and backend themes
- refactored ActiveTheme object to handle the new .active_theme file. Backward compatibility is preserved, but this change introduces a BC because the getActiveTheme method has been replaced with the new getActiveThemeBackend one
- introduced a BC to DeplayerInterface because now it is required an instance of ActiveTheme instead of the Theme
- updated tests