Refactored code that checks settings and dependencies

I'd like to propose slightly modified initialization/validation code for django CMS. Below is an explanation of changes made to current code:

1. Functions `validate_dependencies` and `validate_settings` were moved to new module called `cms.utils.setup`. This allows easy transition from old (pre 1.7) django versions which do not support `AppConfig` and it's ready method. On installations based on django < 1.7 check will be called from `cms.models` on django >= 1.7 from `AppConfig.ready()` method.
1. `setup` functoin was added to allow calling of chechks from one common place.
1. Do not use far to sophisticated `is_installed` helper from `cms.utils.compat.dj` to only chech if some applications is in `settings.INSTALLED_APPS` list/tuple. Introduction of `apps` framework in django does not deprecate `settings.INSTALLED_APPS` or does not make it any worse.
1. Removed check for `cms.middleware.multilingual.MultilingualURLMiddleware` which does not exist.
OK, so this message should be as @digi604 suggested - to remove MultilingualURLMiddleware? If so, should we check for presence of locale middleware and multilingual middleware or just for the second and then raise an exception? Does it matter if there are this two middlewares? IHMO no.