Basic Scene generation: populate with a character

[calummackervoy]

Closes #36

What

• Adds mud:sceneGenerationEndpoint to the MUD configuration
• The client is expected to POST a scene to start with (e.g. the building I selected and my party characters)
• The server will then add a hard-coded character to the scene and return this
• It is then the client's responsibility to render content for the scene to the user

Why

• It allows a mechanism for server-side content to be generated when building a scene (later it will be extended to accept context-type parameters)
• Building the scene is a starting point to build a specific kind of scene - a dialogue (Getting started with Dialogue! #40 )