Fix a number of outstanding issues:

* {{}} escape their contents, {{{}}} and {{& }} do not
* Add support in the parser, tokenizer and AST for partials
  with context (support is still not there in the runtime)
* Fix some inconsistencies with the old behavior involving
  the correct printing of null and undefined
* Add Handlebars.Exception
* Fixed an issue involving ./foo and this/foo
* Fleshed out helperMissing in the specs (this will be
  moved out into handlebars proper once registerHelper
  and registerPartial are added)

lib/handlebars/runtime.js

@@ -70,7 +70,7 @@ Handlebars.Runtime = function(context, fallback, stack) {
 
   var newContext = this.context = new Handlebars.Context;
 
-  if(context.isContext) {
+  if(context && context.isContext) {
     newContext.data     = context.data;
     newContext.fallback = context.fallback;
   } else {
@@ -103,17 +103,22 @@ Handlebars.Runtime.prototype = {
   mustache: function(mustache) {
     var idObj  = this.accept(mustache.id);
     var params = mustache.params;
+    var buf;
 
     for(var i=0, l=params.length; i<l; i++) {
       params[i] = this.accept(params[i]).data;
     }
 
     if(toString.call(idObj.data) === "[object Function]") {
-      this.buffer = this.buffer + idObj.data.apply(this.wrapContext(), params);
+      buf = idObj.data.apply(this.wrapContext(), params);
     } else {
       if(params.length) { throw new Error(mustache.id.parts.join("/") + " is not a Function, so you cannot have Function parameters"); }
-      this.buffer = this.buffer + idObj.data;
+      buf = idObj.data;
     }
+
+    if(buf && mustache.escaped) { buf = Handlebars.Utils.escapeExpression(buf); }
+
+    this.buffer = this.buffer + (buf == null ? '' : buf);
   },
 
   block: function(block) {