Fixed rendering to text and to headers

* Everything is now using string interpolation
* Actions are now correctly rendered in to_header
  for Entity instances
Updated Entity specs

* Fixed code formatting
* Fixed minor typo in #to_header for Entity
* Updated specs for #to_header and #to_text
Merge pull request #32 from arax/rendering_fix

Rendering fixes for Occi::Core::Entity
get_class() method is in a different class