Add changelog

An attempt at a changelog format that will support multiple languages/releases, based on [Grape](https://github.com/intridea/grape/blob/master/CHANGELOG.md).

Part of the Grape contributing process is to include entries in the changelog with each pull request. Since the changelog refers to pull requests IDs, the contributor can't know the proper link before creating the request, so it looks like they tack on another commit afterwards that adds the entry.

Format is:

    # <version> - YYYY/MM/DD
    ## <language>
    ### Features
    ### Fixes

Sections should be added as they are needed, so releases don't end up with empty headings.

Content was populated by comparing [1.0.0 to master](https://github.com/ResourceDataInc/Centroid/compare/rb-1.0.0...master), and including the pull requests that changed code.

Thoughts?