Use prettyPrint `opt_whenDone` callback to resolve a deferred

[mickmcgrath13]

Original issue: canjs/canjs/issues/2706
Dependent on: canjs/bit-docs-html-canjs/issues/189

The prettyPrint method takes a couple of parameters, the first of which is a function to be executed when the prettifying is complete. (See here).

Where we call the prettyPrint method, we could provide a return value of a Deferred which is resolved within prettyPrint's done callback.

Something like:

    require('./prettify-engine');
    require('./prettify.less');
    var $ = require('jquery');
    module.exports = function () {
    	var doneDeferred = $.Deferred();
        var codes = document.getElementsByTagName('code');
        for (var i = 0; i < codes.length; i++) {
            var code = codes[i];
            if (code.parentNode.nodeName.toUpperCase() === 'PRE') {
                code.className = code.className + ' prettyprint';
            }
        }
        prettyPrint(function(){
        	doneDeferred.resolve();
        });
        
        return doneDeferred;
    };