Added basic comparators to Occi::Core classes

All key classes of Occi::Core now have #== and #eql?
explicitly implemented.
Fixed ActionInstance#as_json rendering

* Fixed a minor bug in attribute rendering
* Made specs for #as_json more readable
Extended Category specs to include eql. checks

Checking #==, #eql? and #equal?
Merge pull request #38 from arax/cmp_fix

Added basic comparators to Occi::Core classes