Added support for $app->forbidden(), with accompanying unit tests

Added support for an `$app->forbidden()` handler which functions identically to `$app->notFound()`, but for HTTP 403 Forbidden errors.  Unit tests have also been added, and are passing.

Travis build can be found here:
https://travis-ci.org/mdlayher/Slim/builds/5666420

Also tweaked a few minor lines in `$app->notFound()` which did not conform to PSR-2.