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Added mode (environment) handling

Now the app can distinguish between  'development' and 'production' modes.
Global variable 'app' now available from the views.

Now the views can access the app variable (which is a \Slim\Slim instance). This is mostly to emulate Symfony2's default templating variariables.
Added 404 and 500 error pages

Added not found and error handling functions. Added templates for both actions.