Adding the possibility to configure the strategy within yii's main config file

This has two benefits:
- Keeping the configuration centralized
- Beeing able to use ACL as Git Submodule as there a no changes needed in the module itself.

There are two attributes now for the configuration in main.php:
strategy : The strategy name which should be used (defaults to nestedSet.pathMaterialization)
strategy_config : An array including the strategy config, as it was previously declared in components/strategies/nestedSet/pathMaterialization/config.php

This update keeps full backwards compatibility and it will continoue looking for a config.php in the strategy directory, if no configuration was given.