Add method to return strings to numbers < 10 != 0

This method will be used to construct bigger number strings
afterwards.
Add method to return thousands string

This method returns a string for the thousands digit.
Since it returns either a string or nil, I'll probably require to pad
the original number with additional zeros before I begin to obtain the
strings.
Change output of units into lowercase numbers

To preserve a normalized output from all methods that output strings.
Change output of thousands method to lowercase
Add method to return string for hundreds
Add method that returns string for tens and below
Add method that converts numbers into cardinal words