add GitHub Flavored Markdown to README

* This makes the example code easier to read,
  especially when perusing the docs on github
Fixing double standard with nil handling.
Updates RSpec so that tests run correctly on 1.9.3-p194

* I also had to upgrade jeweler to make it play nicely with my version
  of bundler. Newer versions of jeweler lock to bundler '~>1.0' instead
  of '~>1.0.0'
Regenerate gemspec for version 1.4.0
Fix spacing in README
Fix typo in README
Add .with to README
Remove constants after defining them in examples

This squashes the "already initialized constant" warnings.
Use string interpolation in #instance_variable_set
Use a symbol argument to #instance_variable_set
Use Hash#values_at in .with
Inline local variable
Use Array#== to test fieldwise equality
Extract #values
Use a single condition in #eql?
Rename block parameter

"values" is a more obvious name.
Use Enumerable#zip in constructor
Use #values in #hash
Use Enumerable#map in #hash
Use Enumerable#inject in #hash