feat(libraries) put libraries in a table, link to readme

I think this is more readable, also the link to how-to documentation may be useful

Looks like this:

![image](https://cloud.githubusercontent.com/assets/2231765/5055581/8b4c28c0-6c1a-11e4-8f28-24b6c994967e.png)


I've unabashedly used Bootstrap. I saw there was once a thought of redesigning without Bootstrap (https://github.com/mruby/mruby.github.io/pull/3), is that still active?