Made a first attempt at a Model and Mesh API

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[Mesh]
* The intent of this class is to retain vertices and indices for rendering
  multiple related polygons with materials
[Model]
* A collection of meshes and skeletal data to render game models
Added a GraphicsDeviceService to enable the GamerServicesDispatcher
Added a NetworkSession for communicating with the level editor
Added a ServiceContainer to retain all Service object instances

|SOURCE|
[XNA/GraphicsDeviceService]
* This class manages the GraphicsDevice to ensure that only one is present
[XNA/Renderer]
* The constructor initialises the GamerServicesDispatcher
* Added ClearColour to set the clear colour with a Color object
[MainGame]
* A NetworkSession instance is created to host the level editor network session
* The clear colour is changed upon receiving a clear colour from the level
  editor
[ServiceContainer]
* This retains instances of different object types for instances which require
  a particular service type