Number2Words: Fix some typos and rename the context of the two digits
numers
Number2Words: Add spec for checking for the twenties

Even when the specs are passing I don't feel very sure about this
implementation. The problem is that's there seems to be a lot of
knowledge between the objects involve.
Number2Words: Remove code duplication between numbers using inheritance
Number2Words:
- Add specs for Twenties
- Add more assertions for checking limits of ranges between 1-9 and 11-19