move NoMenuError to error.rb
Merge pull request #3343 from timoschilling/no_menu_error

move NoMenuError to error.rb
refactor DependencyError
rename active_admin to activeadmin

& clean up all the old references to gregbell/*
Merge pull request #3344 from timoschilling/dependency_error

refactor DependencyError
use Arbre 1.0.2