Update changelog and add structure

  * renamed CHANGES.md to CHANGELOG.md
  * move changes from README.md to CHANGELOG.md
  * add structure that follows http://keepachangelog.com/

I am not sure about development details like adding ruby versions to travis and changes on specs.
Anyone has some input on this?

Fixes #54 
Can you remove these headings? We can add them back as we need them. Leave the 'Unreleased' one, just remove the ones up to 'Other'.