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'.
Added .gitignore and plan to improve README
Started to switch to ffi-ncurses
Menu and ScrollList are ok
Going from example to example -- added separate file for 'graph' widget
Working on examples -- up until 'entry_ex.rb'
Examples up to 'scroll_ex.rb'
Made 'tawny-cdk' transition from 'ncurses' gem to 'ffi-ncurses' -- all examples work now
Merged 'creating-gem' branch