Change admin directory

Created by: zamith

Why:

• For existing application this could be problematic, especially with name collisions

This change addresses the need by:

• Allowing the installation directory to be configured in the app, only once and uses that any time a generator is called. It could be configured like this:

Administrate.configure do |config|
  config.admin_directory = "administrate"
end

Re-opened from #330 because I changed the branches in my repo and had to close it.