Refactored validation in Occi::Core::Attributes
Refactored attribute rendering in Occi::Core

* Moved attribute-specific stuff closer to
  Occi::Core::Attributes
Fixed validation of boolean attributes

* #blank? is not enough, since false.blank? is true
Fixed defaults for boolean attributes
Another attempt to fix boolean attr defaults
Merge pull request #36 from arax/rndr_refactor

Refactored Occi::Core::Attributes rendering + small validation fixes