[core] Convert changelog to markdown

This pull request does the following:

* Converts `changelog.txt` to `CHANGELOG.md`
* Updates `rake changelog` to generate changelog in markdown
* Adds `rake github_release` which will create a [github release object](https://github.com/fog/fog/releases) for each release in the changelog (if necessary).

The release information will be accessible via [atom feed](https://github.com/fog/fog/releases.atom)

The `github_release` task treats the `CHANGELOG.md` as the source of truth for release information. The task compares the `CHANGELOG.md` to current github release objects and in the process creates any missing github release objects. 

I have updated the `RELEASE.md` file to indicate that this task should be executed as the last step of the release process, however, it can be executed at any time independently of the release.