Add OpenAI's endopoint-specific classes

[miltondp]

The current class that handles connections to the OpenAI API (GPT3CompletionModel) has too many responsibilities: it handles connections to the Completion and Edits endpoints, and PR #9 adds the ChatCompletion endpoints as well.

We should have classes that are endpoint-specific (such as CompletionModel, EditsModel or ChatCompletionModel), instead of model family specific (GPT3CompletionModel).

[castedo]

#51 might be useful here