piety.py: add tasks function to display information about all tasks
 rename Task class guard attribute to enabled,
 add true and false functions to assign to enabled
 rename tasks variable to schedule, add task_list
 ievent is now a dictionary from event to count (not just total count)
console.py: add command history, access with ^P (previous) and ^N (next)
pysht.py: if value is None, don't print
piety/README.md: change 'guard' to 'enabling condition'
samples: make sure all samples working, tweak docstrings