key capturing, subroutine stack

Added (basic) key capturing capability and a subroutine stack. The key
capturing must be extended to tell whether a key is pressed or
released, and a timer to measure system time between operations must be
added.
pyglet migration, delay/sound timer functionality

Application now uses the Pyglet framework for graphics and keylogging.
Delay/sound timers should count down properly now. Keylogging should
work properly now, except for awaiting input (FX0A).