adding minimal icomoon font with rdf and json-ld logos

Happened to check the playground on Chrome/Android, and the RDF icon looked _terrible_. This wasn't getting reproduced by the in-browser emulator. Not that this is a high-priority environment, but it was hard to even get away from the page to something else.

Quick solution: just created a 2 icon font with the RDF and JSON-LD logos... didn't really need the latter. Really was quite painless, feedback welcome. Here's the [demo](http://bollwyvl.github.io/json-ld.org/playground-more-icons/playground).

I definitely like the icomoon approach to eventually solving #325: totally painless.