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/handlebars_lexer.js

@@ -96,7 +96,10 @@ Handlebars.HandlebarsLexer.prototype.lex = function() {
       return "OPEN_ENDBLOCK";
     } else if(peek === "^") {
       this.getchar();
-      return "OPEN_INVERSE"
+      return "OPEN_INVERSE";
+    } else if(peek === "{" || peek === "&") {
+      this.getchar();
+      return "OPEN_UNESCAPED";
     } else if(peek === "!") {
       this.readchar();
       this.setupLex(); // reset the lexer state so the yytext is the comment only