[1.0.0] Basics for Posts and Categories

PR brings basics for Posts and Categories for discussion

See #1 #2 and #3 

Brought back manifest.json to enable installation under current (not-composer-enabled) CMS 0.4 and 0.5
Not necessary to use 'en' as default. Remove it please. It will be en by default. Otherwise u can use constant from config, but not plain text =)