This adds a method for creating tutorials.  There is a basic tutorial which is mostly defined in a pug template.  Code blocks are user-editable, and will update a live iframe with the results.  The pug template can define tests for code blocks which are run as part of the test suite.

Fixed some inconsistent use of jQuery $.Deferred where sometimes we added the `new` operator before calling it, which is unnecessary.

Simplify the required pug template a little more.

Don't mock webGL in the tests; it needs to be done in the inner iframe.

Allow passing through a crossDomain setting of null.

Simpler tutorial titles.

Order tutorials.