Add Ex 12 of LRTHW.

Add Exercise 12 of Learn Ruby the Hard Way. Introduces 'require'
syntax (for importing Ruby libraries), do loops, block syntax,
the reserved word "p", what looks like an implicit loop scoped
with braces, object methods, and the "open" function.
Add error in ex12.rb to mistakes.rst.

Ruby threw an error when I wrote Exercise 12 from Learn Ruby the Hard
Way as written. It seemed pretty easy to fix, given my experience
with cryptic error messages, so I fixed it.