Added count function

The count function has an if/else statement checking if each element of the array is a string. Is this something you want? If so, what would you like the function to return if it fails? 
The convention I follow or assert messages is a clause or sentence without a capital letter and no period. It is inconsistent with English anywhere else in my code. It is only applied to these messages, so `'throws expected exception'` and `'add lengths'`. Change nothing else.