Add ex2_07.scm
Add ex2_07.rb
Rename ex2_07.scm -> ex2_07-8.scm; add sub-interval
Rename ex2_07-8.scm -> ex2_07-9.scm; add width
Rename ex2_07-9.scm -> ex2_07-10.scm; add spans-zero?
Fix syntax errors in ex2_06.scm