Arena-py

Changed
- Event handler bug fix.

Changed
- Bug fix with turning dictionaries into Material as.

Changed
- Bug fix with get_persisted_obj. Ensures that `persist` is True.

Added
- Allow user defined username and passwords with env vars `ARENA_USERNAME`, `ARENA_PASSWORD`. Need to
specify both to bypass auth.

Changed
- scene callbacks (`on_msg`, `new_obj`, `delete_obj`, etc.) have an `Object` instance as an argument rather than dict.
- `get_persisted_obj` returns an `Object`.

Added
- Support for scene landmarks with the `scene.add_landmark` method.

Changed
- Bug fix with `scene.new_obj_callback` which didn't work well with code that updated `scene.users`.

Added
- Support for user callbacks (`user_join_callback`, `user_left_callback`).
- Use these callbacks to handle when a user joins and leaves, respectively.
- `scene.users` will give now you a dictionary of users, mapping `object_id`'s to `Camera` objects.
- Add Changelog.md to keep track of changes.

Changed
- `Arena` class renamed to `Scene`, but using `Arena` is still allowed. Updated examples to reflect this.
- Tiny bug fix with `update_objects`.