* {{}} 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)