Fixed JSON rendering of actions for entities

Actions available for the given entity should always be rendered
as arrays of type identifiers not full JSON representations.
Fixed action link parsing for resources

When parsing a resource link with '?action=' included in URI,
link is parsed as Occi::Core::Action and not Occi::Core::Link.
Merge pull request #77 from arax/entity_actions_json_fix

Fixed JSON rendering of actions for entities