adds the following string methods with specs: ljust, rjust, center

Hi there,
I have added String#center, String#ljust, String#rjust and specs.
I tried to be as DRY as possible implementing ljust, use ljust to implement rjust and than use both to implement center.

If you have suggestions please give me feedback, I am sure this code could be improved. 

I am looking forward to to start contributing this great project!

Cheers