Fixed stress bug

An indentation error in the stress() function would cause the program
to crash when attmepting to stress mark 4 letter words.
Added trainingError() and stringFrequency()

Added two new functions for calculating the error made in the training
mini tests and for calculating the average number of times a unique
string occurs. Also streamlined the numberOfUniqueWords() function with
list comprehensions and set().
Updated experiment 2 parameters

Changed to a theta of 3, fixed an $overused_words bug, changed the
prompt-to-retype message, which now presents in red.