Bundle capistrano-ext/multistage with the main Gem.

Almost everybody uses the multistage extension, and having it in a
sepate gem has never really made that much sense. With this commit you
can use multistage by requiring `capistrano/ext/multistage`. The Gem
`capistrano-ext` no longer required.

lib/capistrano/ext/multistage.rb

@@ -0,0 +1,62 @@
+require 'fileutils'
+
+unless Capistrano::Configuration.respond_to?(:instance)
+  abort "capistrano/ext/multistage requires Capistrano 2"
+end
+
+Capistrano::Configuration.instance.load do
+  location = fetch(:stage_dir, "config/deploy")
+
+  unless exists?(:stages)
+    set :stages, Dir["#{location}/*.rb"].map { |f| File.basename(f, ".rb") }
+  end
+
+  stages.each do |name|
+    desc "Set the target stage to `#{name}'."
+    task(name) do
+      set :stage, name.to_sym
+      load "#{location}/#{stage}"
+    end
+  end
+
+  on :load do
+    if stages.include?(ARGV.first)
+      # Execute the specified stage so that recipes required in stage can contribute to task list
+      find_and_execute_task(ARGV.first) if ARGV.any?{ |option| option =~ /-T|--tasks|-e|--explain/ }
+    else
+      # Execute the default stage so that recipes required in stage can contribute tasks
+      find_and_execute_task(default_stage) if exists?(:default_stage)
+    end
+  end
+
+  namespace :multistage do
+    desc "[internal] Ensure that a stage has been selected."
+    task :ensure do
+      if !exists?(:stage)
+        if exists?(:default_stage)
+          logger.important "Defaulting to `#{default_stage}'"
+          find_and_execute_task(default_stage)
+        else
+          abort "No stage specified. Please specify one of: #{stages.join(', ')} (e.g. `cap #{stages.first} #{ARGV.last}')"
+        end
+      end 
+    end
+
+    desc "Stub out the staging config files."
+    task :prepare do
+      FileUtils.mkdir_p(location)
+      stages.each do |name|
+        file = File.join(location, name + ".rb")
+        unless File.exists?(file)
+          File.open(file, "w") do |f|
+            f.puts "# #{name.upcase}-specific deployment configuration"
+            f.puts "# please put general deployment config in config/deploy.rb"
+          end
+        end
+      end
+    end
+  end
+
+  on :start, "multistage:ensure", :except => stages + ['multistage:prepare']
+
+end