Add {{log}} and {{debugger}} helper for HTMLBars.

Requires https://github.com/tildeio/htmlbars/pull/112 (will need to update `bower.json` once merged).
`this` is actually the view, not the context. In HTMLBars helpers it should always be the view.

I'm unsure there is anywhere you can say "give me the context" any more. I suggest just changing the description and `Logger.info` to say this references the view.