Add Changelog generation to our process.

[crtrott]

We want to start a changelog using an automatic process. I went with this:
https://github.com/skywinder/github-changelog-generator

You need to install a fairly new ruby though (and I had to install openssl-devel) but other wise really simple to use. Here are the settings I used (replace token with one you generate):

github_changelog_generator kokkos/kokkos --token XXX --no-pull-requests --include-labels 'bug,enhancement,Feature Request' --enhancement-labels 'enhancement,Feature Request'

I then moved the tags one up each (since we close the issues after the push to master).
And you can see the result here:

https://github.com/crtrott/kokkos/blob/changelog/CHANGELOG.md

[crtrott]

I believe this is good enough for now, we might want to do some hand tuning of the titles, and we can go with partial lists (one can provide a tag from which to start) for future updates, which we copy manually into the changelog.