Added context manager support for all main tmux objects:

- `Server`: Automatically kills the server when exiting the context
- `Session`: Automatically kills the session when exiting the context
- `Window`: Automatically kills the window when exiting the context
- `Pane`: Automatically kills the pane when exiting the context

Example usage:

```python
with Server() as server:
    with server.new_session() as session:
        with session.new_window() as window:
            with window.split() as pane:
                pane.send_keys('echo "Hello"')
                # Do work with the pane
                # Everything is cleaned up automatically when exiting contexts
```

This makes it easier to write clean, safe code that properly cleans up tmux resources.