Shells API #17

This is to be one of the main hooks for Python. A shell is something that the user interacts with, generally through a keyboard. A shell should have the following characteristics:

1. Ability to delegate to other shells.
2. High level API for receiving user input from the keyboard.
3. Direct access to a Communicator object for sending data back to the client.
4. Fine-grained control over how the command prompt operates. (Supported by high-level API, of course)