Improve gem structure

Hello,

I took the liberty and made some changes to the structure used by this gem adding some conventions used in other gems with C extensions:

- ext, lib, and test folder structure
- Rely on rake-compiler for compilation
- Upgrade to latest conventions of RubyGems, Rake and test task
- Use a simple `Gemfile` to bootstrap local development

While I'm not fond to Bundler usage, last point might help others that want to work on this library.

This structure helps set the foundation for better cross-compilation support too.

Hope you accept this and thank you for taking the time and maintain this library!

:heart: :heart: :heart: