Fixed markup 

Fixed markup for configuration of on_unauthorized_access
fix misspelling in CHANGELOG
simplify AssetRegistration
cleanup ActiveAdmin::Application#namespace
Merge pull request #3437 from timoschilling/simplify_asset_registration

simplify AssetRegistration
Merge pull request #3433 from macarthy/patch-2

Fixed markup
Merge pull request #3434 from timoschilling/patch-1

fix misspelling in CHANGELOG
Merge pull request #3438 from timoschilling/cleanup_application_namespace

cleanup ActiveAdmin::Application#namespace