Add requires
Add files that require everything in a chapter
Move IRB sessions into chapter subdirectories
Extract combinator constants into ski_combinator.rb
Extract partial recursive functions into partial_recursive_functions.rb
Add encoding comments where required by Ruby 1.9
Add Gemfile
Install Treetop
Add README
Add minimum Ruby version to README
Extract FizzBuzz code to fizzbuzz.rb
Extract FizzBuzz helpers to helpers.rb
Add README to Programming with Nothing directory
Demonstrate lambda calculus parser and operational semantics in README
Use #puts in main README for consistency
Use snake_case for all file and directory names

This is more consistent and less surprising for Rubyists.
Move per-chapter files to top level

Less typing required in IRB.
Group files within chapters

Where there's more than one idea in a chapter, this makes it easier to
see which files belong to which idea.
Remove warnings from README IRB sessions