Optional hints

Pass information from an experimental URL parameter back to Blockly for the experiment in which users may have to press a button to see the hint.  Changes to Blockly are in https://github.com/code-dot-org/blockly/pull/757
Preceding three lines could be a nice helper e.g. `try_get_parameter('stanford_experiment')`

If you'd prefer to work with a single-level hash rather than `[]`-wrapped values there's `Rack::Utils.parse_query URI("http://example.com?par=hello&par2=bye").query`, [more details](http://stackoverflow.com/a/9398264/136134)