Add Exercise 2.8 to ex2_07.rb
Rename ex2_07.rb -> ex2_07-8.rb
Fix comments in ex2_07-8.rb
Add Exercise 2.9 to ex2_07-8.rb
Rename ex2_07-8.rb -> ex2_07-9.rb
Add protected spans_zero? method, use for ZeroDivisionError in Exercise 2.10
Rename ex2_07-9.rb -> ex2_07-10.rb