Merge pull request #477 from sergeyklay/master

Some fixes
Fixed issue #463 : Added missing hidden fields to page form
Fixed issue #463 : Add missing hidden fields to blog form
Fixed issue: Fix post image upload issue during update, temporary hack.
New Feature: Add Assets::popup() helper method for adding popup assets
Update feature: Add subtitle to json response
Fixed issue: Cleanup taxonomy from
Fixed issue: Fixed validation message for taxonomy
Fixed issue: Added missed validation messages for post
Fixed issue: Cleanup validation messages
Updated translation: Fixed validation messages, added new translation