Revert Paperclip::Storage::Filesystem#flush_writes

This commit makes this method move by default instead of copying, which
should make it perform significantly better with large files. It falls
back to the old semantics if the mv fails.

lib/paperclip/storage/filesystem.rb

@@ -36,9 +36,13 @@ module Paperclip
       def flush_writes #:nodoc:
         @queued_for_write.each do |style_name, file|
           FileUtils.mkdir_p(File.dirname(path(style_name)))
-          File.open(path(style_name), "wb") do |new_file|
-            while chunk = file.read(16 * 1024)
-              new_file.write(chunk)
+          begin
+            FileUtils.mv(file.path, path(style_name))
+          rescue SystemCallError
+            File.open(path(style_name), "wb") do |new_file|
+              while chunk = file.read(16 * 1024)
+                new_file.write(chunk)
+              end
             end
           end
           unless @options[:override_file_permissions] == false