curated list of libraries

this replaces the widgets page, which never seemed quite right

caveats:

* I'm not sure the current selection provides a sufficiently broad overview - anything else worth adding?
* for the moment, I refrained from adding any kind of search or filtering functionality - browsers' full-text search is just fine for that, at least while the list is still relatively small
* styling is pretty crude - perhaps @rstrangh and/or @mechanoid can improve this a little?

![roca](https://cloud.githubusercontent.com/assets/3515/4213837/1b1dacb4-38bb-11e4-8adc-7ca88e6dff4d.png)