Add :checkout option to allow checkinable pod when using --no-clean

The svn downloader uses export instead of checkout by default since
CocoaPods/CocoaPods#473.

But having a "checkin-able" pod is still usefull for the Pods developpers even
if it has been aggreed to be a minority-use case.
(from CocoaPods/CocoaPods#245 discussion).

lib/cocoapods-downloader/subversion.rb

@@ -3,7 +3,7 @@ module Downloader
     class Subversion < Base
 
       def self.options
-        [:revision, :tag, :folder]
+        [:revision, :tag, :folder, :checkout]
       end
 
       def options_specific?
@@ -23,7 +23,8 @@ def checkout_options
       executable :svn
 
       def download!
-        output = svn!(%|#{export_subcommand} "#{reference_url}" "#{target_path}"|)
+          subcommand = options[:checkout] ? checkout_subcommand : export_subcommand
+          output = svn!(%|#{subcommand} "#{reference_url}" "#{target_path}"|)
           store_exported_revision(output)
       end
 
@@ -41,6 +42,10 @@ def export_subcommand
         result = 'export --non-interactive --trust-server-cert --force'
       end
 
+      def checkout_subcommand
+        result = 'checkout --non-interactive --trust-server-cert --force'
+      end
+
       def reference_url
         result = url.dup
         result << '/'       << options[:folder] if options[:folder]