Fixes EGI-FCTF/rOCCI-cli#7, fixes EGI-FCTF/rOCCI-core#16

Updated class handling for custom classes. Instances of custom
classes are now correctly recognized by kind_of? or is_a? as
related to Occi::Core::Resource and Occi::Core::Entity.
Added spec to cover the parsing of custom resources
Merge pull request #17 from arax/kind_of_fix

Fixes EGI-FCTF/rOCCI-cli#7, fixes EGI-FCTF/rOCCI-core#16