Several enhancements.

- Gemify the file so it can be easily packaged up and re-used. The name
  of the gem is mercadopago-sdk, since another gem called mercadopago
  already exists (see https://rubygems.org/gems/mercadopago and
  https://github.com/kauplus/mercadopago)
- Configuration yml file added to store client credentials.
- Gemfile and Rakefile added to support Bundler and Rake.
- Rename and similar refactorings to make the code more idiomatic.
- Separate version.rb file broken out so the gemspec could refer to it
  without having to load other required files.
- RestClient constants changed from attr_accessor to true constants.
- assert_equal calls have arguments flipped so expected failure messages
  show properly.