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)

spec/tokenizer_spec.rb

@@ -36,12 +36,19 @@ describe "Tokenizer" do
     result[1].should be_token("ID", "foo")
   end
 
-  it "tokenizes a path as 'OPEN ID CLOSE'" do
+  it "tokenizes a path as 'OPEN (ID SEP)* ID CLOSE'" do
     result = tokenize("{{../foo/bar}}")
     result.should match_tokens(%w(OPEN ID SEP ID SEP ID CLOSE))
     result[1].should be_token("ID", "..")
   end
 
+  it "tokenizes a path with this/foo as OPEN ID SEP ID CLOSE" do
+    result = tokenize("{{this/foo}}")
+    result.should match_tokens(%w(OPEN ID SEP ID CLOSE))
+    result[1].should be_token("ID", "this")
+    result[3].should be_token("ID", "foo")
+  end
+
   it "tokenizes a simple mustahe with spaces as 'OPEN ID CLOSE'" do
     result = tokenize("{{  foo  }}")
     result.should match_tokens(%w(OPEN ID CLOSE))
@@ -60,6 +67,11 @@ describe "Tokenizer" do
     result.should match_tokens(%w(OPEN_PARTIAL ID CLOSE))
   end
 
+  it "tokenizes a partial with context as 'OPEN_PARTIAL ID ID CLOSE'" do
+    result = tokenize("{{> foo bar }}")
+    result.should match_tokens(%w(OPEN_PARTIAL ID ID CLOSE))
+  end
+
   it "tokenizes a partial without spaces as 'OPEN_PARTIAL ID CLOSE'" do
     result = tokenize("{{>foo}}")
     result.should match_tokens(%w(OPEN_PARTIAL ID CLOSE))