Fixed word consturctor + some other things

[artog]

Added:

• empty symbol (epsilon)
• whitespace trimming
• empty word is string is null or ""
• copy constructor,
• empty word as static var

Changed:

• toString() to use String.format
• tail() to return a copy of the tail

[Oliv95]

Kan man inte få null pointer exception i Word constructorn när man använder replaceAll

[artog]

You have the correct!