Add support for scenes

[IvarMatstoms]

Implemented parts of /scene api. Added:

• Post to AbstractSmartHomeHub (required by /scene//trigger)
• New class Scene with attributes scene_id, name and icon
• Scene.trigger() to activate a scene
• Hub.get_scenes()
• Hub.get_scene_by_id(scene_id)

[IvarMatstoms]

Tests added, currently testing:

• dict_to_scene
• Scene.trigger

[Leggin]

Looks good thank you!