Add JSON schema for JSON-LD documents

Squashed commit of the following:

commit c915635e3563bca6cae98c941784cac4259814c6
Merge: fe71c94 e16b086
Author: Markus Lanthaler <mark_lanthaler@gmx.net>
Date:   Tue Mar 18 10:09:27 2014 +0100

    Merge branch 'master' of https://github.com/madskristensen/json-ld.org into PR334

commit e16b08696050fcbc778af6224b700c732bb9e81c
Author: Mads Kristensen <post@madskristensen.net>
Date:   Mon Mar 17 18:29:23 2014 -0700

    Created /schema/ and moved the file there

commit 457f72580f9cb10bf0ac8df1a540043389c919e5
Author: Mads Kristensen <post@madskristensen.net>
Date:   Mon Mar 17 06:50:07 2014 -0700

    Fixed based on code review

    I also removed the "id" property since it doesn't resolve.

commit 764d6c70473b97cf8c694b266f3c6cb0c511f22e
Author: Mads Kristensen <post@madskristensen.net>
Date:   Sun Mar 16 09:40:33 2014 -0700

    Minor fix and formatting tweaks

commit 1e4a76a13765f8bb4e99e730b5c89987123c591f
Author: Mads Kristensen <post@madskristensen.net>
Date:   Sun Mar 16 09:05:21 2014 -0700

    JSON schema for JSON-LD documents

    This will allow JSON editors to provide statement completion and
    tooltips for authors of JSON-LD documents.

    I validated all the examples of JSON-LD I could find using
    http://json-schema-validator.herokuapp.com/

This closes #334