store reference to containing question in JME scope

I couldn't think of a better way of letting an HTML object created in JME get hold of the containing question.
Question.scope has a property `question` which is a reference back to the question object. This lets JME functions access the question, without appealing to global variables or passing a question object down the whole evaluation chain.

runtime/scripts/jme.js

@@ -1054,6 +1054,7 @@ var fnSort = util.sortBy('id');
  * @property {object} variables - dictionary of {@link Numbas.jme.token} objects
  * @property {object} functions - dictionary of arrays of {@link Numbas.jme.funcObj} objects. There can be more than one function for each name because of signature overloading.
  * @property {objects} rulesets - dictionary of {@link Numbas.jme.Ruleset} objects
+ * @property {Numbas.Question} question - the question this scope belongs to
  *
  * @param {Numbas.jme.Scope[]} scopes - List of scopes to combine into this one. Scopes appearing later in the list override ones appearing earlier, in case variable/ruleset names conflict.
  */
@@ -1070,6 +1071,7 @@ var Scope = jme.Scope = function(scopes) {
 
 	for(var i=0;i<scopes.length;i++) {
 		var scope = scopes[i];
+        this.question = scope.question || this.question;
 		if(scope) {
 			if('variables' in scope) {
 				for(var x in scope.variables) {

runtime/scripts/question.js

@@ -48,6 +48,7 @@ var Question = Numbas.Question = function( exam, xml, number, loading, gscope)
 	q.originalXML = q.xml;
 	q.number = number;
 	q.scope = new jme.Scope(gscope);
+    q.scope.question = q;
 	q.preamble = {
 		'js': '',
 		'css': ''