test for ENV variables before require added_methods

Using Brice without AddedMethods results in this error while loading the rails console. 
> LoadError: cannot load such file -- added_methods [/Users/ben/.rvm/gems/ruby-1.9.3-p125/gems/activesupport-3.2.12/lib/active_support/dependencies.rb:245:in `load']

That was caused by ```Brice::DSL.brice(package)``` calling ```Brice::DSL.brice_require``` independent of running the configuration block in ```lib/brice/rc/010_added_methods.rb```. 

This pullrequest fixes this by scoping the ```brice 'added_methods' do |config| …``` in ```lib/brice/rc/010_added_methods.rb``` with a test for required environment variables.