Implement TODOs

* Use "Hashie::Mash" instead of "OpenStruct" (though it adds a runtime
  dependency, it is ultimately more flexible, and consistent across Ruby
  versions, than "OpenStruct")
* Update gem description so that it properly renders on "RubyGems.org"
* Include feature-list in "README.md"
* Add better example-code to "README.md"
* Update TODOs