String methods

[defeated]

Hi!

I went to use "something".diff_by_char() and realized that String had only been extended with the diff() method so far.

Added:

"something".diff_by_char("something else")
"something".diff_by_word("something else")
"something".diff_by_line("something else")

(And a few related specs.)

(Note: Also minor update to spec_helper.rb so it stops warning about deprecated rspec2 syntax.)