Refactor core action to return a parser instance based on the response body

Here's another perspective... If someone _really_ didn't want to use excon with conduit-rails, for example if they were already using another connection library. All they have to do is override 2 things. The `Conduit::Core::Connection` class, and the request objects `perform_request` method. Honestly, that's a pretty small price to pay for swapping out an entire connection library. I feel that conduit-rails adhering to the conduit default of excon is pretty reasonable, considering you're replacing an entire core component. However, integration overrides in my opinion are left to the developer.