calculate fingerprints efficiently on large files using Digest::MD5.file()

The tests would broken for StringIO, because it doesn't have a real "path". I've modify the patch to make the test stop complaining [@sikachu]

This closes #470

lib/paperclip/attachment.rb

@@ -225,9 +225,13 @@ module Paperclip
     end
 
     def generate_fingerprint(source)
-      data = source.read
-      source.rewind if source.respond_to?(:rewind)
-      Digest::MD5.hexdigest(data)
+      if source.respond_to?(:path) && source.path && !source.path.blank?
+        Digest::MD5.file(source.path).to_s
+      else
+        data = source.read
+        source.rewind if source.respond_to?(:rewind)
+        Digest::MD5.hexdigest(data)
+      end
     end
 
     # Paths and URLs can have a number of variables interpolated into them