Added support for :styles option being defined as a Proc.

(cherry picked from commit 5799558cb4211bed0fba67df36510a29cfbbd94c)

lib/paperclip/attachment.rb

@@ -32,6 +32,7 @@ def initialize name, instance, options = {}
       @path              = options[:path]
       @path              = @path.call(self) if @path.is_a?(Proc)
       @styles            = options[:styles]
+      @styles            = @styles.call(self) if @styles.is_a?(Proc)
       @default_url       = options[:default_url]
       @validations       = options[:validations]
       @default_style     = options[:default_style]

test/attachment_test.rb

@@ -217,6 +217,18 @@ def thumb; "-thumb"; end
     end
   end
 
+  context "An attachment with :styles that is a proc" do
+    setup do
+      rebuild_model :styles => lambda{ |attachment| {:thumb => "50x50#", :large => "400x400"} }
+
+      @attachment = Dummy.new.avatar
+    end
+
+    should "have the correct geometry" do
+      assert_equal "50x50#", @attachment.styles[:thumb][:geometry]
+    end
+  end
+
   context "An attachment with :url that is a proc" do
     setup do
       rebuild_model :url => lambda{ |attachment| "path/#{attachment.instance.other}.:extension" }