Add submitted by column and fill in the GitHub users that had their PRs merged recently

Since the website is community driven, we don't offer support for the
boxes linked and the idea here is to know which user has added the box
so we can reach out in case someone is having trouble with it.

References: https://github.com/garethr/vagrantboxes-heroku/issues/222#issuecomment-27309058

/cc @garethr @DrOctogon @hyamamoto @FooBarWidget @jiko @ColinHebert