Migrators

I noticed in the Todo list in the readme, you had importers/exporters. While I agree exporters may not be needed since it's open-source and you can always just back up the database, I thought an importer would be very useful.

In fact, we're switching from WordPress to Jekyll/Juvia, and needed to import all our WP comments from the past few years. So, I went ahead and wrote an actual importer.

I based this on the [Jekyll importers](https://github.com/mojombo/jekyll/wiki/blog-migrations), but had to customize it a lot. Jekyll's importers are command-line based, which makes sense for Jekyll, since it's primarily a command line tool. But, Juvia is primarily web-based, with the initial user creation happening through the browser. So, I integrated this one with the web interface as well.

I made it so that you don't have to install the gems needed unless you actually plan on using the import. If you do try to import without installing the gems, I customized the error message to tell you what to do.

And finally, I made it modular and easy to extend and add other platforms from which to import, with WordPress just being the first one, since it's what I needed.

Let me know if you have questions or anything.