Fixed class attributes and class names

* Fixed inheritance for class attributes (from Entity down)
* Fixed class names for os_tpl and resource_tpl
Fixed attribute readers for Occi::Infrastructure

* Using simplified Hashie::Mash syntax
Fixed attribute readers for Occi::Core

* Using simplified Hashie::Mash syntax
Merge pull request #29 from arax/entity_attributes

Fixed entity attributes and mixin class names