Created a gem for `tawny-cdk`.

To do so, I changed the source a little:

* Created `gemspec` and `Gemfile`, adding `lib` to load path
* Changed all `require_relative` inside `lib` to `require cdk/`,
  assuming `lib` on the load path.
* Changed `CDK.version` format to rubygems' default - previously
  it returned '0.8 - 0' but now it should return '0.8.0'.

If you want the gem on Rubygems (and the commodity of installing
it with `$ gem install cdk`), create an account there, go to the source
root and do `rake release`.

Also, the gem name is `cdk` and not `tawny-cdk`.
What do you think of it?
The name is available in Rubygems, so I urge you to claim it soon.