Simplified the action definition/constructor.

Because of the line->block transition in the language, action objects
can't be built out of lines. Instead, an action will take a command
list as a direct argument to its constructor.
Added docstring to enum constructor.
Cleanup of tokenize.py

Added comments throughout, better encapsulated functionality in
tokenize_stream(), fixed several typos in the state machine, and
made the enums more uniform.