New View, Postmeta, Termmeta components

• Added Singleton pattern trait to be used in all classes which should be defined only once
• Updated single-feature classes to use Singleton trait.
• View component is totally rebuilt and now implements Theme Wrapper technique. To wrap a template you need to call $this->extends('..view name...'); at the beginning of your view file
• View render() method is renamed as include() method, this is more logical
• Removed custom fields getters from model class (because of misunderstanding inside the code when used it). Created new classes Postmeta, Termmeta with similar features. To get some custom field you will need to write something like this: $fields = new Postmeta(); echo $fields->my_field;