Rename variable "line" to "words"

Due to the fact that variable name "letters" is used instead of "word" in the
Word class, the variable name in the Line class is also changed for
consistency.
Use Iterator interface for both Line and Word classes

This change is aimed to avoid reusing methods that does the same thing. For
example, the setLetterColor method in the Word class is unnecessary because
the foreground color of the Letter can be set by using the next() method and
then using the setForegroundColor from the returned Letter instance.
Add JavaDoc for split method
Implement delete method