print warning when including Capybara::DSL globally

lib/capybara/dsl.rb

@@ -2,6 +2,15 @@ require 'capybara'
 
 module Capybara
   module DSL
+    def self.included(base)
+      warn "including Capybara::DSL in the global scope is not recommended!" if base == Object
+      super
+    end
+
+    def self.extended(base)
+      warn "extending the main object with Capybara::DSL is not recommended!" if base == TOPLEVEL_BINDING.eval("self")
+      super
+    end
 
     ##
     #