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.
Sorry @mmarzantowicz, actually the check currently implemented in develop is completely wrong!
My fault as I just checked that the moved implementation works the same as the original one, but we should just tell to remove `MultilingualURLMiddleware`.
This is message used to be for the people upgrading from old version of django CMS to the one with MultilingualURLMiddleware, now the goal is the opposite: we want people upgrading from 2.3 to 3.0 to remove MultilingualURLMiddleware