Supply actual index to add_utterance rather than defaulting to 1
Always include referent, even if set to None, when creating a new utterance
Document new_utterance utility function
Add example section for creating a new dialogue & adding 2 utterances
Outline get_utterance function
Use get_utterance function to return a specific utterance via API
Add _links section to JSON returned by utterance API function
Add self link to get_dialogue API route
Move _links construction to beginning of function