Put the helper methods into Paperclip::Task module

Closes #434

lib/tasks/paperclip.rake

-def obtain_class
+module Paperclip
+  module Task
+    def self.obtain_class
       class_name = ENV['CLASS'] || ENV['class']
       raise "Must specify CLASS" unless class_name
       class_name
-end
+    end
 
-def obtain_attachments(klass)
+    def self.obtain_attachments(klass)
       klass = Paperclip.class_for(klass.to_s)
       name = ENV['ATTACHMENT'] || ENV['attachment']
       raise "Class #{klass.name} has no attachments specified" unless klass.respond_to?(:attachment_definitions)
@@ -13,6 +15,8 @@ def obtain_attachments(klass)
       else
         klass.attachment_definitions.keys
       end
+    end
+  end
 end
 
 namespace :paperclip do
@@ -23,8 +27,8 @@ namespace :paperclip do
     desc "Regenerates thumbnails for a given CLASS (and optional ATTACHMENT and STYLES splitted by comma)."
     task :thumbnails => :environment do
       errors = []
-      klass = obtain_class
-      names = obtain_attachments(klass)
+      klass = Paperclip::Task.obtain_class
+      names = Paperclip::Task.obtain_attachments(klass)
       styles = (ENV['STYLES'] || ENV['styles'] || '').split(',').map(&:to_sym)
       names.each do |name|
         Paperclip.each_instance_with_attachment(klass, name) do |instance|
@@ -37,8 +41,8 @@ namespace :paperclip do
 
     desc "Regenerates content_type/size metadata for a given CLASS (and optional ATTACHMENT)."
     task :metadata => :environment do
-      klass = obtain_class
-      names = obtain_attachments(klass)
+      klass = Paperclip::Task.obtain_class
+      names = Paperclip::Task.obtain_attachments(klass)
       names.each do |name|
         Paperclip.each_instance_with_attachment(klass, name) do |instance|
           if file = instance.send(name).to_file(:original)
@@ -60,8 +64,8 @@ namespace :paperclip do
 
   desc "Cleans out invalid attachments. Useful after you've added new validations."
   task :clean => :environment do
-    klass = obtain_class
-    names = obtain_attachments(klass)
+    klass = Paperclip::Task.obtain_class
+    names = Paperclip::Task.obtain_attachments(klass)
     names.each do |name|
       Paperclip.each_instance_with_attachment(klass, name) do |instance|
         instance.send(name).send(:validate)