integrate list of open source education projects

From conversation with @DBness about open-source education-related projects (not including ones already in the README:
- https://p2pu.org/
- http://www.raspberrypi.org/resources/
- https://www.gitbook.io/

And more from @pedroha:

> Canvas seems to be making big impact in the LMS space (also listed in Aidan's oss-watch)
> http://www.instructure.com/
>
> Codecombat is a fun learning game
> http://codecombat.com/contribute
> http://codecombat.com/contribute/archmage
>
> Mozilla WebMaker tool (to teach web development: Mozilla Thimble live html editor)
> https://support.mozilla.org/en-US/kb/contribute-webmaker-code
> http://thimble.webmaker.org
>
> Mozilla Labs
> https://mozillalabs.com/en-US/projects/
>
> Mozilla
> https://wiki.mozilla.org/Webdev/GetInvolved
>
> Mozilla FirefoxOS
> https://developer.mozilla.org/en-US/Firefox_OS/Developing_Gaia
>
> Mozilla PKI
> https://developer.mozilla.org/en-US/docs/PKI
>
> Fun (but not trivial for beginner coders). Minecraft in Javascript
> http://voxeljs.com/